TCP/IP Networking Terms you ll need to understand: Techniques you ll need to master:

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1 5 TCP/IP Networking Terms you ll need to understand: Subnet mask Subnetting Classless Interdomain Routing (CIDR) Transmission Control Protocol/Internet Protocol (TCP/IP) Address Resolution Protocol (ARP) Reverse Address Resolution Protocol (RARP) Hot Standby Routing Protocol (HSRP) Telnet Ping File Transfer Protocol (FTP) Techniques you ll need to master: Describing IP address classes Identifying TCP/IP functions Identifying the use of Network Address Translation (NAT) Explaining TCP/IP application services 1

2 2 Chapter 5 This chapter delves into many of the commonly confused topics within the world of internetworking. Mastery of these topics is essential for CCIE candidates; these technologies will serve you well in your daily activities. We begin with the most common protocol, TCP/IP, and we ll use it as the base for our more advanced discussions throughout this chapter and the remainder of the book. The following CCIE blueprint objectives as laid out by the Cisco Systems CCIE program are covered in this chapter: Addressing Classless Interdomain Routing (CIDR), subnetting, Address Resolution Protocol (ARP), Network Address Translation (NAT), Hot Standby Router Protocol (HSRP) Services Domain Name System (DNS), Bootstrap Protocol (BOOTP), Dynamic Host Configuration Protocol (DHCP), Internet Control Message Protocol (ICMP) Applications Telnet, File Transfer Protocol (FTP), Trivial File Transfer Protocol (TFTP) Access Lists Standard access lists and extended access lists, including where and how to place and design them As with other chapters in this book, we have provided additional information in this chapter for both completeness and in preparation for additional subjects as the CCIE program expands. This will allow you to use this book as a reference source throughout the CCIE certification process and beyond. TCP/IP Overview Transmission Control Protocol/Internet Protocol (TCP/IP) is by far the most popular networking protocol in use today. The Internet links many different hardware types, and TCP/IP enables the various hardware types to communicate effectively with each other. Figure 5.1 shows the TCP/IP protocol suite and how it maps to the seven-layer OSI model. When using TCP/IP in the OSI model, the Transport layer (TCP or UDP) provides connection orientation (TCP) or connectionless services (UDP), and the Network layer (IP) provides best-effort delivery (connectionless). The next section describes what makes up an IP address and the associated addressing schemes available. Later in this chapter, we ll take a closer look at TCP s functions. The Internet Protocol (IP) was described by Jon Postel in RFC 791 in September The following URL provides you with some of the most common RFC s are available: 459/index.shtml.

3 TCP/IP Networking 3 Application Presentation Telnet File Transfer BOOTP, DHCP, Protocol (FTP) TFTP, NTP Session Transport Network Data Link Physical TCP provides connection-oriented delivery UDP provides connectionless delivery OSPF, RIP, IGEP/EIGRP, BGP ICMP Ethernet T/Ring FDDI (802.3) (802.5) (ANSI X3T9.5) ATM Figure 5.1 OSI-TCP/IP model. IP Addressing Review The network layer addressing used by IP is a field 32 bits in length and represented in a dotted decimal format, such as IP addresses have three defined portions: a network portion, a host portion, and a subnet mask. A subnet mask (also a 32-bit field) is used to identify and distinguish between the network and host portions, as discussed later in this chapter. Figure 5.2 demonstrates a typical network address, using a Class A IP address. In Figure 5.2, the number 10 represents a network portion, and the numbers represent the host portion. Together, these two portions form the IP address, which is 32 bits in length. 32 bits Network Host bits 8 bits 8 bits 8 bits Figure 5.2 A typical IP address.

4 4 Chapter 5 Table 5.1 IP address classes. Address Class Range Default Subnet Mask Class A 1 through Class B 128 through Class C 192 through Class D 224 through Class E 240 through 255 Reserved The original RFC classified IP addressing into five main classes. Table 5.1 lists the RFC s IP address classes. Using the table, you can see that the IP address shown in Figure 5.2 is a Class A address. The address range is reserved for loopback devices. For example when you read the Cisco documentation CD-ROM on your PC, the address used is to indicate the local CD-ROM drive. Class D addresses are reserved for Multicast groups. The address is reserved for broadcasts. By applying a default mask, as shown in Table 5.1, to an IP address, the IP model is known as the classful model. IP routing protocols that use Table 5.1 s definitions are referred to as classful routing protocols (for example, RIP v1). This is contrasted with routing protocols that use a mask other than the default. These types of routing protocols are known as classless routing protocols (for example, OSPF). Class D addressing is reserved for multicast groups. For example, the Cisco IP routing protocol Enhanced Interior Gateway Routing Protocol (EIGRP) sends multicast hello packets to the multicast address Class E addressing is reserved for future use. A simple way to observe an IP address s class is to look at the first couple of bits in the IP address s first octet. The value contained within the first few bits will tell you what class of IP address you are working with: 0 Class A network 10 Class B networks 110 Class C networks 1110 Class D networks Class E networks You can clearly see how the bit pattern indicates the class of the IP address, as shown in Figure 5.3.

5 TCP/IP Networking 5 Bits required bits Ranges Class A 0 Network Host bits Class B 1 0 Network Host bits Class C Network Host Figure 5.3 Bit patterns for Class A, Class B, and Class C addressing. Note: A Cisco router will apply the longest match rule when deciding where an an IP packet will be sent. Consider the case in which a router can have two or more different next hop addresses for the same network. The router will choose the next hop that has the longest mask that matches the destination network. This is called the longest match rule. Due to the rapid growth of hosts on the Internet or intranets (not public networks), it was soon evident to the Internet community that IP addressing would eventually be depleted. Therefore, to allow for the continued expansion of the Internet, subnetting was implemented to allow IP administrators to maximize the use of an IP address space. Subnetting Subnetting allows the network or IP address administrator to maximize the use of an IP address space within the network. A subnet mask borrows bits from an IP address s host portion and uses the bits to define new networks. If subnetting is implemented, IP addresses have three sections: Network Subnet (new) Host Address Note: All IP addresses have a mask associated with them, either implied (default) or defined. There are three address representations: dotted decimal, bitcount, and hexadecimal. The subnet or network defines the arbitrary segmentation performed by the network administrator. The subnet allows the creation of a hierarchical routing network.

6 6 Chapter 5 A subnet mask is a 32-bit decimal number that is used to identify a network and its host addresses. Subnet masks can be the classful kind, as shown earlier in Table 5.1. Class A, B, and C network addressing schemes are not much use in today s complex Internetworks unless you extend the mask or use variable length subnetting to avoid wasting IP address space. The phrase variable length subnet mask (VLSM) refers to the fact one network can be configured with different subnet masks. For example, a network could be configured to have one mask that allows only two hosts and another mask that can be extended to allow 512 hosts. VLSM ensures that IP addressing is not wasted. Think of a serial line that contains two routers. Why assign a Class C address for two nodes? Instead, you could assign an address (subnet) that contains only two hosts; the mask can be used to accomplish this. To determine the number of hosts or subnets available on a network, you need to examine the IP addresses in binary. To determine the number of hosts or subnets you can assign to a network, you apply the formula 2 n -2, where n equals the number of borrowed bits. Why are two subnets subtracted? Because one address is reserved to identify the subnet and the other is used to send broadcasts (bits that are set to all 0s or all 1s are used for broadcasts). Determining how many hosts or subnets you can assign to a network is best explained with examples. Let s assume the subnet mask has been applied to your network. How many subnets are available when assigning the subnet address of ? Looking at the subnet mask, you can interpret the 240 as in binary. Hence, 4 bits have been borrowed from the host portion of the IP address to form a subnet. Therefore, the subnet mask formula would be 2 4-2, which equates to 14 subnets (2*2*2*2=16-2=14). Why do we take away 2 subnets? The reason is that they are used to represent the subnet and the broadcast address. Bits that are set to all 0s are the network (wire address) and all 1s are used for broadcasts. This can be shown in the following: In this subnet and subnet mask, note that: Network address First usable host address Last usable host address Directed Broadcast address Broadcast address

7 TCP/IP Networking 7 Similarly, consider the mask How many end nodes could reside on the network? Note that 192 in binary is Hence, two bits have been borrowed, so the end nodes can use the last six bits. The formula would be 2 6-2, which equates to 62 hosts. Note: When using a 26-bit subnet mask, you need to use the ip subnet command to access all of the subnets that the mask allows! It is vital that you have a good understanding of how an IP address network and host portion is calculated. You should be able to calculate the number of hosts on a network using any IP addressing scheme. For additional review, let s look at a couple more examples of how to calculate the host and subnet portion of any given class of address. Given the host address of /24, what is the subnet and broadcast address? You need to know what a network address such as /24 means. In this example, the address is the equivalent of the network with a subnet mask of , or 24 bits of subnetting. The notation /24 means that the subnet mask uses 24 bits, or the equivalent of a subnet mask represented as in dotted format. Therefore, /24 is the same as In binary, is: And the mask, in binary is: Performing a logical AND operation on the host address and subnet mask will provide you with the subnet mask, which has been derived as , or a Class C address. To determine the subnet, you must perform a logical AND function on the host. Logical AND means that 1 and 1 equates to 1 only. The remaining options are 0 AND 0 is 0, 0 AND 1 is 0. The logical AND operation provides the following: IP ADDRESS Subnet Mask EQUALS NETWORK

8 8 Chapter is a subnet address. To determine the broadcast address, you need the decimal equivalent of all one bits ( ), which is 255; hence, the broadcast address of a subnet is Finally, let s look at a Class A host address of Using a Class C mask, what is the network portion and how many hosts can reside on this network? The logical AND function is performed once more. A Class C mask is when represented in decimal format. Therefore, and in binary is A logical AND between the address and mask yields: indicates a subnet of The number of hosts available on a Class C mask is 2 8-2, or 254 hosts, because 2 addresses are used to identify the subnet and the directed broadcast address. A directed broadcast address is sent to all hosts on the subnet only. ( is the subnet and is a directed broadcast address for all users on the local subnet.) Table 5.2 provides a useful guide that can help you to prepare for the exam. Table 5.2 displays the decimal value and binary value of a subnet number followed by the number of available subnets. The number of hosts that can reside on each subnet follows. Note: Try some subnet examples on your own and then compare them to a subnet calculator freely available on the Internet. Cisco s Web site ( techtools/ip_addr.html) has a subnet calculator. Table 5.2 Common subnets. Decimal Subnets Hosts 252 ( ) 64 subnets 2 hosts 248 ( ) 32 subnets 6 hosts 240 ( ) 16 subnets 14 hosts 224 ( ) 8 subnets 30 hosts 192 ( ) 4 subnets 62 hosts 128 ( ) 2 subnets 126 hosts

9 TCP/IP Networking 9 Now that we ve covered the IP addressing and the formats that are used to represent IP addresses, let s move on to a more advanced IP routing concept Classless Interdomain Routing (CIDR). Classless Interdomain Routing (CIDR) In the past few years, the expansion of the Internet has been phenomenal. Currently, the Internet uses more than 70,000 routes. From 1994 through 1996 the routing table was increased from around 20,000 entries to more than 42,000. How can network administrators reduce the large routing table size? Each routing entry requires memory and a table lookup by the router each time a packet is required to reach a destination. Reducing memory requirements and the time it takes to send a packet to the destination provides faster response times for packets to travel around the Internet. Classless Interdomain Routing (CIDR) helps to reduce the number of routing table entries and memory requirements. CIDR helps to conserve resources, because it removes the limitation of using the default mask (which wastes IP address space) and leaves the addressing up the IP designer. CIDR is used by routers to group networks together in order to reduce routing table size and memory requirements. CIDR is typically represented with the network number/bits used in the mask, such as /24, or the equivalent of Now that we ve covered CIDR and the purpose of CIDR, let s move on to how devices such as PCs map layer 2 addresses to layer 3 addresses using Address Resolution Protocol (ARP) and Reverse Resolution Protocol (RARP). ARP and RARP Address Resolution Protocol (ARP) and Reverse Resolution Protocol (RARP) carry out important functions in the TCP/IP model, which allows devices to communicate at layer 2 of the OSI model. Remember, all frames are sent to a valid MAC address. So, before one IP host can communicate with another, the source device must have an identified layer 2 address to traverse the physical medium or use broadcast frames to locate resources on any particular physical media. ARP is used when a source device needs to know the destination s layer 2 MAC address to allow communication between two devices. ARP is a layer 2 frame sent as a broadcast frame with a known IP address requesting the destination s MAC address. For example, you might Telnet to a local router with a known IP address, such as Your PC does not have a layer 2 address or MAC address to send the frame to, so ARP obtains the MAC address. For example, ARP is used between a Client PC and a Cisco router for the Telnet application protocol. In contrast, RARP is used when a source device knows a destination s

10 10 Chapter 5 MAC address but the IP address is unknown. RARP obtains the unknown IP address. Typically, RARP is used with diskless workstations where the workstations send out requests for IP addresses with a known local MAC address. Figure 5.4 shows the ARP/RARP frame format. The function of each field in an ARP and RARP frame is described as follows: Hardware Type Specifies the hardware in use. For example, this value is set to 1 for Ethernet or 6 for IEEE 802 networks. Protocol Indicates the protocols in use. For example, 0800 is used to indicate IP. Length of Hardware Address Indicates the length of layer 2 addresses, 48 bits. Length of Protocol Address Defines length of protocol addresses. For example, for IP this field is set to 4 bytes (32 bits). Operation Code Defines whether the frame is an ARP or RARP. 1 is an ARP request, 2 is an ARP reply, 3 is a RARP request, and 4 is a RARP reply. Sender Hardware Address Identifies the sender s layer 2 MAC address (48 bits). Sender Protocol Address Identifies the sender s IP address (32 bits). 32 bits Length of Hardware Address Hardware Type Length of Protocol Address Protocol Operation Code Sender Hardware Address Sender Hardware Address Sender Protocol Address Sender Protocol Address Target Hardware Address Target Hardware Address Target Protocol Address Note: Hardware addresses are 48 bits ( ) and protocol address are 32 bits in length. Figure 5.4 ARP/RARP frame format.

11 TCP/IP Networking 11 Target Hardware Address Specifies a destination s address. In an ARP request, this field is set to a broadcast of FF-FF-FF-FF-FF-FF (48 bits). Target Protocol Address Specifies a destination s layer 3 address (32 bits). The ARP and RARP protocol will provide IP-aware devices with valuable information needed to successfully send data across a network, regardless of hardware types. You should ask yourself what MAC address will a local device use to send an IP packet to a remote station, which may have many routers in between. Most commonly, a local device will use the local gateway or the local router s MAC address. To display the IP ARP table on a router, you issue the IOS show ip arp command, as shown in Listing 5.1. Listing 5.1 The show ip arp command. R1>sh ip arp Protocol Address Age (min)hardware Addr Type Interface Internet c3b.ed6d ARPA Ethernet0 Internet c3b.ed6d ARPA Ethernet0 Internet c07.ac00 ARPA Ethernet0 Internet c3b.ed6d ARPA Ethernet0 Internet dc.b736 SNAP TokenRing0 Internet f53.5cff ARPA Ethernet0 Listing 5.1 provides the IP address and the associated MAC address used to reach a device. To clear the ARP cache on a Cisco router, you use the clear arp command. Note: If your router is configured for other routable protocols, such as AppleTalk, the show arp command will also display AppleTalk ARP entries. To display only IP ARP entries, use the show ip arp command. Remember, there is also another type of ARP used in Frame Relay networks Proxy ARP. Proxy ARP maps the DLCI to the remote IP address, essentially a layer 2 to layer 3 mapping in Frame Relay. We will now look at how a network can use Network Address Translation (NAT) to connect to the Internet, even though the network does not have a registered address. Network Address Translation (NAT) To reduce the impact of network address depletion due to the rapid growth of the Internet, many large IP networks needed the ability to retain their current addressing scheme yet be able access the Internet. This can be accomplished with Network Address Translation (NAT) defined in RFC 1631.

12 12 Chapter 5 The implementation of NAT by the Cisco IOS supports most of the applications we have discussed so far, including Domain Name System and File Transfer Protocol. The common applications supported on Cisco routers running NAT include the following: Routing table updates (OSPF, RIP, and so on) HTTP, DNS zone transfers, TFTP, BOOTP, telnet, SNMP, finger, NTP, NFS, rlogin, rsh, rcp ICMP, FTP (including PORT and PASV commands), NetBIOS over TCP/IP. The following URL provides a full list of supported applications: 792_pp.htm#xtocid11070 NAT is a standard defined in RFC Cisco devices started supporting NAT in IOS versions 11.2 and higher. NAT grants the ability to retain a network s original IP addressing scheme while translating that scheme to valid Internet IP addresses. Thus the layer 3 address is changed when the packet is sent out to the Internet and vice versa. NAT Terminology To clarify this discussion and to fully prepare for the exam, you should review some of the terminology used in a NAT environment. You need to understand a number of terms when using NAT, most notably: Inside Local Address An IP address assigned to a host on the internal network and is not being advertised to the Internet. This address is generally assigned by a local administrator. This address is not a legitimate Internet address. Inside Global Address A registered IP address as assigned by InterNIC. Outside Local Address The IP address of an outside host of the network that is being translated. Outside Global Address The IP address assigned to a host on the outside of the network that is being translated. NAT Operation on Cisco Routers When a packet leaves an inside network, NAT translates the inside address to a unique InterNIC address for use on the outside network, as shown in Figure 5.5. The router in Figure 5.5 will be configured for an address translation and will maintain a NAT table. When the packet returns from the outside network, the NAT router will again perform an address translation from the valid InterNIC address to a local inside address.

13 TCP/IP Networking 13 Inside local IP addresses Inside Network Outside Network PC Ethernet E0 Cisco Router R1 S0 Internet Hosts PC TR1 Inside Address NAT Table Outside Address Inside global IP address assigned by InterNIC, /24 InterNIC has assigned you the address to /24 (This pool of NAT addresses is CCIE.) Figure 5.5 NAT overview. Let s look at the steps required to configure NAT on a Cisco router. The basic configuration tasks are as follows: 1. Determine the network addresses to be translated. 2. Configure inside network with the IOS ip nat inside command. 3. Configure the outside network with the IOS ip nat outside command. 4. Define a pool of addresses that will be translated with the following IOS command: ip nat pool <pool-name> <start ip address> <end ip address> <mask> 5. Define the addresses that will be allowed to access the Internet with the following IOS command: ip nat inside source list <access list number> pool <pool name>

14 14 Chapter 5 Now, for a more specific illustration, let s configure NAT on Router R1 in Figure 5.5, the NAT pool name is going to be CCIE (you can use any name). Let s assume that InterNIC has assigned you the Class C address of /24. Your service provider has also supplied you the unique address /24 to use on your serial connection. Listing 5.2 provides a sample NAT configuration for this setup. The listing assumes that your setup has an IP routing protocol that advertises the /24 and /24 IP networks. Listing 5.2 Sample NAT configuration. hostname R1 ip nat pool CCIE netmask ip nat inside source 1 pool CCIE interface e0 ip address ip nat inside interface tokenring0 ip address ip nat inside interface serial 0 ip address ip address secondary ip nat outside access-list 1 permit access-list 1 permit Listing 5.2 s configuration will translate the inside addresses /24 and /24 into the globally unique addresses in the range /24. Monitoring NAT To monitor the operation of NAT, you can use the following commands: show ip nat translation [verbose] show ip nat statistics The show ip nat translation command displays the current active transaction. The show ip nat statistics command displays NAT statistics, such as how many translations are currently taking place. NAT can also support many other advanced features, such as TCP load distribution. See the Need to Know More Section at the end of this chapter for additional sources of information.

15 TCP/IP Networking 15 Note: TCP load distribution is typically used in large IP networks that have server farms. A server farm contains two or more servers that are typically critical to a high end users. You might want to distribute network TCP traffic across many servers but only use one IP address. TCP load distribution will ensure all servers are equally loaded. TCP load distribution is sometimes referred to as Port Address Translation (PAT). PAT basically uses the same IP address, but different port addresses. Hot Standby Router Protocol (HSRP) HSRP is a protocol that allows networks to provide a virtual default gateway. Through HSRP you create a virtual default gateway address that is shared by multiple routers. To illustrate how HSRP can provide default gateway support, refer to Figure 5.6. In Figure 5.6, you can see a network with two local routers configured with an Ethernet interface address of /24 for Router R1 and /24 for Router R2. Notice that both routers share a common Ethernet network. All IP Backbone S0 S0 R1 R HSRP IO IO / / /24 Default gateway (for all hosts on this network) Figure 5.6 Example without HSRP.

16 16 Chapter 5 devices in Figure 5.6 have been configured with a default gateway pointing to Router R1. If Router R1 goes down or the Ethernet interface becomes faulty, all the devices must be manually reconfigured to use the second default gateway (Router R2 Ethernet address). HSRP enables the network administrator to elect one of the two routers to act as the default gateway. If the elected router goes down, the second router assumes the IP default gateway. The IOS command under the Ethernet interface, standby track <interface of WAN> will allow the router to monitor the WAN link. If the WAN link continuously fails past a threshold, the HSRP default router will decrease its priority to allow a more reliable WAN connection to provide a gateway. For example in Figure 5.6, if the link on R1 to the WAN fails past a threshold then R2 will assume the HSRP address to provide a faster connection to the IP backbone network. As shown in Figure 5.6, having redundant routers is helpful, but hosts can get confused because they are typically only allowed one default gateway. When using HSRP, as shown in Figure 5.7, both routers retain their unique Ethernet addresses. However, HSRP allows them to share a virtual address. This virtual address is assigned to each host as its default gateway. In the event of a router failure, the other will assume control of the virtual address. IP Backbone S0 S0 R1 Virtual MAC Address C-07-AC-01 R2 E0 MAC Address (physical) /24 00-D0-97-D /24 E0 MAC Address (physical) 00-D0-97-D Default gateway Default gateway MAC Address is 00-C0-0C-C1-AC-01 Figure 5.7 Example Using HSRP.

17 TCP/IP Networking 17 Configuring HSRP You can configure certain HSRP parameters to elect a default gateway router and monitor routers WAN links. To illustrate, let s configure HSRP on Routers R1 and R2 shown in Figure 5.6. Let s make Router R1 the default gateway, because the link on Router R2 is occasionally prone to WAN problems. All PCs on the network are configured to use the default IP gateway /24. Listing 5.3 displays the configuration for Routers R1 and R2. Listing 5.3 HSRP configuration on R1 and R2. Hostname R1 interface Ethernet0 ip address standby priority 120!Highest priority wins standby preempt standby ip !HSRP address used by local devices standby track Serial0! Monitor serial 0 for failures Hostname R2 ip address standby priority 110 standby preempt standby ip standby track Serial0 In Listing 5.3, Router R1 will be the active default gateway, because it has the higher priority. If Router R1 fails, Router R2 will assume the virtual IP address of Monitoring HSRP To monitor HSRP settings, you can use the show standby command. The show standby command displays a router s state and which router is active. Listing 5.4 displays the output from the show standby command. Listing 5.4 The show standby command. Ethernet0 - Group 0 Local state is Active, priority 120, may preempt Hellotime 3 holdtime 10 Next hello sent in 00:00: Hot standby IP address is configured Active router is local Standby router is expires in 00:00:08 Tracking interface states for 1 interface, 1 up: Up Serial0

18 18 Chapter 5 Listing 5.4 is taken from Router R1, and it displays R1 s local state as active. This means that Router R1 has assumed the role of the default gateway. Other valid router states are speak (negotiating who will become the default gateway, highest priority will win) and standby (backup router). Theoretically, end users will not see any significant network outage if one of the routers or interfaces becomes unreachable. The ARP protocol will resolve the new Virtual MAC address and hosts will start to use R2 as the default gateway. HSRP is supported over Token Ring and ATM LANE (IOS 11.2+) networks. Now that we ve reviewed some of the functions of the IP layers and how you can provide a resilient network, let s look at a typical Transport Control Program session running over the IP layer, including how a typical TCP session is started and terminated. Transport Control Protocol (TCP) TCP runs over IP and provides guaranteed delivery of packets to their destination. Let s examine how this protocol behaves in a typical network connection (Chapter 2 covers the TCP segment format in detail). With interactive TCP data flow, the flags are vital. Using TCP Flags The flag fields in a TCP segment contain important details regarding how network devices, such as how routers or other devices, should handle the TCP segment or UDP datagram. Table 5.3 presents the 6 bits in a TCP segment that are commonly referred to as flags. Now, let s examine a typical Telnet connection startup, data transfer, and session closure between a PC and a Cisco router. Figure 5.8 shows a typical TCP session. Reviewing a typical TCP session will help you to see how TCP flags are set and changed as a session is started and shut down. The steps in a TCP Telnet session are: 1. A PC sends a request with the SYN bit sent to 1. The destination port number will be 23 (Telnet). The PC will also place an initial sequence number (such as, ) in the segment; this is a random number generated by the PC.

19 TCP/IP Networking 19 Table 5.3 Flag URG (U) ACK (A) PSH (P) RST (R) SYN (S) FIN (F) Flags in a TCP segment. Function (Urgent) Informs the other station that urgent data is being carried. The receiver will decide what do with the data. (Acknowledge) Specifies a number to use to synchronize segment flows between devices. (Push) Informs the end station to send data to the Application layer immediately. (Reset) Resets an existing connection. (Synchronize) Initiates a connection, commonly known as established. (Finished) Indicates that the sender is finished sending data and terminates the session. 2. The router responds with its own sequence number (such as, ) and acknowledges (ACK) the segment sent by the PC. 3. The PC sends a segment that acknowledges (ACK) the routers reply. Note: The first 3 steps are commonly known as the TCP three-way handshake. 4. Data is transferred. 5. The PC completes the data transfer and closes the Telnet session by sending a TCP segment with the FIN flag set to The router acknowledges (ACK) the request with an acknowledgement. 7. At this stage, the session is still open and the router could send data (this is known as TCP half close), but it usually doesn t send data. Instead, the router usually sends a segment with the FIN bit set to The PC acknowledges the request routers FIN request, and the Telnet session is closed. You need to know the TCP process and how packets are sequenced and acknowledged. TCP acknowledgements specify the next expected segment from a sender. A TCP session requires three segments to start and four to shut down. You can see in Figure 5.8 that whenever a device requests a TCP session, the SYN bit is set to 1. If you are concerned about unauthorized users from the Internet, then you should not allow sessions to initiate (originate) from the Internet. In other words, do not only allow Telnet sessions (or any other for that matter) that have the SYN set to 1 from outside of your network.

20 20 Chapter 5 PC Step 1 PC requests a Telnet session. Flags U A P R S F Destination Port is 23 or Telnet. Inital sequence is Ack set to 0. Step 3 Flags U A P R S F Sequence is Ack Connection Request (SYN) Connection Reply (ACK and SYN) PC acknowledges Router (ACK) Step 4 Data Flow Router Step 2 Router responds with its own sequence number, and acknowledges the segment by increasing the PC sequence number by one. Flags U A P R S F Source port is 23. Ack is Its own sequence is Step 5 Flags U A P R S F Step 8 PC acknowledges request. PC tears down session (FIN) (ACK) (FIN) (ACK) Step 6 PC acknowledges request. Step 7 Router also tears down connection. Flags U A P R S F Note: It takes 3 TCP segments to open a telnet session and 4 TCP segments to close it. Figure 5.8 A typical TCP session.

21 TCP/IP Networking 21 Other TCP Functions TCP is a vast topic and it is impossible to cover it in one chapter (entire books have been written about TCP). Table 5.4 summarizes some of the main functions of TCP. For further study, see the Need to Know More section for some TCP resources. Let s now discuss some of the application services provided by TCP/IP, including how TCP/IP users can access remote devices, how diskless workstation can boot over an IP network, and how name resolution can be handled. TCP/IP Services The TCP/IP protocol provides a number of services that allow users to connect to local or remote hosts. Specifically, these services include: Domain Name System (DNS) Bootstrap Protocol (BOOTP) Dynamic Host Configuration Protocol (DHCP) Internet Control Message Protocol (ICMP) The following sections cover each of these services along with the IOS commands used to implement them on Cisco routers. Table 5.4 Other TCP functions. Function Name Description TCP Half Close Allows one station to receive data, even though it has completed data transfer. Simultaneous Open Specifies the state when two stations simultaneous try to connect to each other. TCP detects this event and shuts down one of the sessions. Simultaneous Close Specifies the event when two stations close at the same time. TCP handles this situation. Slow Start Monitors the rate at which segments are sent and acknowledged to make sure no segments are lost in intermediate parts of the network. The window size is gradually increased; hence, the name slow start. Nagle Algorithm States that only one TCP connection can have only one outstanding packet to be acknowledged at any time. The Nagle algorithm is turned off by default on a Cisco router. To turn it on, apply the service Nagle command.

22 22 Chapter 5 DNS In a large IP environment, network users need an easier way to connect to hosts without having to remember 32 bit IP addresses that s where the Domain Name System (DNS) comes into play. DNS provides a service that allows users to use a host s name in place of an IP address in order to connect to hosts. When DNS services are running the host s name is used to request its IP address from a DNS server (a host running the DNS service) and do the translation for the user transparently. In other words the user never sees this request and host to IP address translation. The client simply connects to a host name, and a DNS server does the translation. To illustrate, let s look at the following Cisco router command that provides a host lookup for users connected via the console or virtual ports (note a router will not provide DNS server responses to client devices such as PC s or Unix hosts): ip host Router Using the preceding command, the router will automatically translate the name Router2 to the IP address of when requested. The host name is used in place of the IP address. This is the basic function of DNS. DNS runs over UDP and uses port number 53. Bootstrap Protocol (BOOTP) BOOTP provides the ability to for diskless PCs to download their operating system across the network. The use of BOOTP is becoming increasingly uncommon in today s modern network, however this aging technology is still required to be part of a CCIE s knowledge. By default, Cisco routers do not forward broadcasts. BOOTP requests are sent as layer 2 broadcasts; hence, by default, BOOTP requests will be dropped by a Cisco router. BOOTP can be supported across routers using the IOS interface configuration command ip helper-address <address of bootp server>. BOOTP runs over UDP using port 67 on the server and 68 on a client. When the helper address is configured, Cisco routers forward data on the following ports: TFTP requests port 69 DNS queries port 53 Time requests port 37 NetBIOS name service port 137 NetBIOS datagram service port 137

23 TCP/IP Networking 23 BOOTP 68 (to server) TACAS port 49 To forward a specific port, you use the syntax ip forward-protocol {udp [port]}. Dynamic Host Configuration Protocol (DHCP) Dynamic Host Configuration Protocol (DHCP) enables TCP/IP clients to request certain parameters, such as their IP address, mask, and default gateway from a server offering the DHCP service. DHCP reduces the common scalability issues of configuring every single host on a network with an IP address. DHCP is an extension of the BOOTP protocol. To use DHCP, a TCP/IP client sends out a broadcast to request its IP address, and the DHCP server (a device running DHCP, such as a Windows NT server) replies with the requested parameters, such as the default gateway and subnet mask. If a DHCP server does not exist on the local network, you can configure a Cisco router to relay these broadcasts. By default, a broadcast will not be forwarded by a router. However, the IOS ip helper-address <server ip address> command will relay the broadcast to a specified unicast address, the DHCP server (as discussed in the preceding section). Internet Control Message Protocol (ICMP) The Internet Control Message Protocol (ICMP) provides a number of useful services that are supported by the TCP/IP protocol, including ping requests and replies. Ping requests and replies enable an administrator to test connectivity with a remote device. ICMP also provides feedback control in the form of messages to end devices. ICMP is described by RFC 792. Be aware that ICMP runs over IP, which means that there is no guarantee of delivery (because IP is a connectionless protocol). Listing 5.5 provides a sample ping command in which an administrator wishes to see if a remote device is reachable by sending the remote device a ping request from a Cisco router. By default a Cisco router will send out a series of five ICMP requests whenever the ping command is issued. Listing 5.5 The ping command. R2>ping Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to ,!!!!! Success rate is 100 percent (5/5), R2>

24 24 Chapter 5 The exclamation point (!) indicates a successful reply. The ping command can also advise you that the end device is not reachable via a special code character, as depicted in Table 5.5. There are three other common applications that use TCP/IP, namely Telnet, File Transfer Protocol (FTP), and Trivial File Transfer Protocol (TFTP). These services use well-known TCP ports, as follows: Telnet TCP port 23 File Transfer Protocol TCP ports 20 and 21 Trivial File Transfer Protocol or (TFTP) runs over IP/UDP and uses port number 69. Telnet Telnet runs over TCP and provides the ability to connect to and manage remote devices. When you connect from one router to another, you are using the Telnet protocol, which uses port number 23 in the TCP header. TCP is connectionorientated protocol, hence telnet is an application that guarantees a reliable service to the end user. FTP The File Transfer Protocol (FTP) allows users to transfer files from one host to another. Two ports are required for FTP one port is used to open the connection (port 21) and the other port is used to transfer data (20). FTP runs over TCP and is connection oriented. TFTP The Trivial File Transfer Protocol (TFTP) allows users to send files without the extra overhead of TCP. TFTP runs over UDP. There is no guarantee of data Table 5.5 Possible IOS codes when using the ping command. Code Description! Each exclamation point indicates the receipt of a reply.. Each period indicates the network server timed out while waiting for a reply. U Destination unreachable. N Network unreachable. P Protocol unreachable. Q Source quench. M Could not fragment.? Unknown packet type.

25 TCP/IP Networking 25 delivery, and the Application layer is responsible for resending any lost frames. When you copy an image from a TFTP server to a local router, you can use TFTP. TFTP uses port 69 in the IP header. Be aware of common port numbers and the connection methods used by Telnet, FTP, and TFTP. Let s make sure you have absorbed all this information in your quest to become a CCIE by testing you with some challenging practice questions about TCP/IP.

26 26 Chapter 5 Practice Questions Question 1 What is the default priority when using HSRP on a Cisco router? a. 10 b. 110 c. 100 d. Must be configured The correct answer is c. The default priority used on Cisco routers is 100. You can override this by using the IOS standby priority <1-255> command. Answers a and b are incorrect, because, while they can be set manually, they are not the default. Answer d is incorrect, because default parameters do not need to be configured. Question 2 View the following display: Ethernet0 - Group 0 Local state is Active,priority120,maypreempt Hellotime 3 holdtime 10 Next hello sent in 00:00: Hot standby IP address is Active router is local Standby router is Tracking interface states for 1 interface: Up Serial0 What is the HSRP state of this router, priority, and the HSRP address? a. Standby,120,may preempt b. Active,120, c. Active,100, d. Active,120,

27 TCP/IP Networking 27 The correct answer is d. The display shows that the state of the router is Active (Local state is Active), the priority is set to 120 (priority120), and the HSRP address is (Hot Standby IP address is ). Answer a is incorrect, because the standby IP address is not supplied. Answer b is incorrect, because the standby address is not Answer c is incorrect, because the IP address is invalid. Question 3 What is the destination port number used in a TFTP file transfer? a. 23 b. 69 c. 21 d. 161 The correct answer is b. TFTP uses the well-known port of 69. Answer a incorrect, because port 23 is used by Telnet. Answer c is incorrect, because port 21 is used by FTP. Answer d is incorrect, because SNMP uses UDP port 161. Question 4 You send a ping request from your local router, and the reply you receive is the message U. What does this mean? a. The end device is unreachable. b. The end device could not fragment your request. c. The end device is not configured for IPX. d. Both answers b and c are true. The correct answer is a. Cisco routers use several symbols to abbreviate various conditions. U is used to notify the administrator that the end device is unreachable. Answer b is incorrect; the symbol M would be returned by the router if the packet could not be fragmented. Answer c is incorrect, because the option is an invalid error code used by the router it is possible to send ping requests to IPX devices. Answer d is incorrect because b and c are false. Earlier in the chapter, Table 5.3 describes a full list of possible codes returned by a Cisco router.

28 28 Chapter 5 Question 5 Which of the following services is connection orientated? [Choose the two best answers] a. FTP b. Telnet c. TFTP d. IP The correct answers are a and b. FTP and Telnet run over TCP, which is connection orientated. Answers c and d are incorrect, because TFTP and IP are connectionless services. Question 6 What service is used to translate names to IP address? a. NAT b. DMZ c. DNS d. DHCP The correct answer is c. The Domain Name Service (DNS) is used to translate names to IP addresses. Answer a is incorrect; NAT is used for address translation at layer 3. Answer b is incorrect, because DMZ, or demilitarized zone, is not part of NAT s terminology. Answer d is incorrect, because DHCP is used for IP address leases. Question 7 What well-known TCP port does Telnet use? a. 21 b. 23 c. 99 d. 20

29 TCP/IP Networking 29 The correct answer is b. Telnet uses port 23. Answers a and d are incorrect, because FTP (not Telnet) uses ports 20 and 21. Answer c is incorrect, because 99 is not a common port number used by TCP. Question 8 What is the subnet and broadcast address for the following device with an IP address of /27? a. subnet is and broadcast I b. subnet is and broadcast is c. subnet is and broadcast is d. Not enough data supplied The correct answer is b /27 is the equivalent of / The decimal 224 in binary is , so you have 5 bits for hosts minus 2 for broadcasts. Therefore, the range of addresses total 30 or from to The broadcast address is 31 or, in binary, the last six bits are Answer a is incorrect; the subnet is correct, but the broadcast address is not Answer c is incorrect, because is a host address and not a subnet address. Answer d is incorrect, because enough information is provided to satisfy the question. Question 9 You require an IP network that has, at most, 62 hosts. What subnet mask will accomplish this requirement? a b c d The correct answer is d. The number of hosts available can be deduced by solving the value of n (the number of bits) to satisfy the equation 2 n -2=62. That value is 6, because 2 6-2=64-2=62 hosts. Hence, the mask requires 2 bits from the host portion, or (192). The subnet mask that will allow this is Answer a is incorrect, because it will allow 254 hosts. Answer b is incorrect; it will allow 30 hosts. Answer c is incorrect; it will allow 16,382 hosts.

30 30 Chapter 5 Question 10 In what fields does the IP checksum calculate the checksum value? a. Data only b. Header and data c. Header only d. Not used in an IP packet The correct answer is c. The checksum in the IP header only checks the IP header information. The data is not checked. Upper layers, such as TCP, will need to support this function when required. The TCP segment checks both the header and data fields. Answers a and b are incorrect, because the data is not checked. Answer d is incorrect, because there is a checksum performed on the IP header. Question 11 How many hosts are available on a Class B network? a. 254 b. 16,382 c. 65,534 d. 16, e. More information required The correct answer is c. The default mask on a Class B network is Hence, 16 bits are available for hosts =65,536 minus 2 host addresses for broadcasts leaves 65,534 hosts. Answer a is incorrect; more than 254 hosts can reside on a Class B network. Answer b is incorrect; 16,382 does not satisfy any class of address (that is, class A, B, C, D, or E). Answer d is incorrect, because the number 16,777,214 represents the number of hosts available with a Class A address. Answer e is incorrect, because enough information is provided so that you can deduce that answer c is correct. Note: You will not permitted to use calculators during the exam. You should know the number of hosts available in each network class.

31 TCP/IP Networking 31 Question 12 What versions of IOS support NAT? [Choose the two best answers] a b c d e The correct answers are d and e. Network address translation was first supported in 11.2 and subsequently will be supported in version Answers a, b, and c are incorrect, because NAT is not backward compatible. Question 13 You are using NAT to translate your local addressing to a unique address on the Internet. What IOS command will place a network on the inside? a. ip nat-inside b. inside nat c. ip nat inside d. ip inside nat The correct answer is c. The correct syntax is ip nat inside. Beware of similar questions on the examination where the dash is placed in the incorrect position. Answers a, c, and d are incorrect, because they are not the correct command syntax needed to configure NAT.

32 32 Chapter 5 Question 14 Your local users who run BOOTP are complaining that they can no longer obtain their operating systems since the BOOTP server was moved off their local segment. The server now resides on the network /24 with a host address of /24. What IOS command will forward not only BOOTP requests but other requests, such as DHCP? a. ip forward udp 68 b. ip helper address c. ip helper-address d. ip helper-address e. ip helper-address The correct answer is e. The correct syntax to forward BOOTP requests across a router is ip helper-address Answer a is incorrect; this command will only forward BOOTP requests and not DHCP. Answer b is incorrect, because the dash is missing between the helper and address keywords. Answer c is incorrect, because this command will send the requests to another host, which is not the BOOTP server. Answer d is incorrect, because the command is configured for the wrong network ( ). Question 15 Your local users who run BOOTP are complaining that they can no longer obtain their operating systems since the BOOTP server was moved off their local segment. You do not know the host address of the server but you know the server resides on the same network, /24. What IOS command will still allow BOOTP stations to load their operating system? a. ip forward udp all b. ip helper address c. ip helper-address d. ip helper-address e. ip helper-address The correct answer is c. Answer c will send all requests to the network where the server resides, but it will be to the address , which is the broadcast address for the network. So, all devices will check to see if they can respond to

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