Data & Computer Communications. Lecture 8. Network Layer: Logical addressing. In this lecture we will cover the following topics:

Transcription

1 Data & Computer Communications MSCEG 425 Lecture 8 Network Layer: Logical addressing Fall Overview In this lecture we will cover the following topics: 14.Network Layer: Logical addressing 14.1 IPv4 Addresses 14.2 IPv6 Addresses 14.3 Summary (part 14) 2

2 Position of Network Layer 3 Duties of Network Layer Internetworking Logically connecting heterogeneous networks to look like single network to upper transport and application layers. Addressing Each device (a computer or a router) over the Internet must have unique and universally accepted address. Routing Packet can not choose its route to the destination. The routers connecting LANs and WANs make this decision. Packetizing The network layer encapsulates datagram/segments received from upper layers and makes packets out of them. Fragmenting Each router de-capsulates the IP datagram from the received frame, process it and encapsulates it into another frame. 4

3 14.1 IPv4 ADDRESSES An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a device (for example, a computer or a router) to the Internet. Topics discussed in this section: Address Space Notations Classful Addressing Classless Addressing Network Address Translation (NAT) 5 Note An IPv4 address is 32 bits long. Note The address space of IPv4 is 2 32 or 4,294,967,296. 6

4 Note The IPv4 addresses are unique and universal. 7 Dotted-decimal notation and binary notation for an IPv4 address Binary Notation Dotted-Decimal Notation Identifier used in network layer to identify each device connected to the Internet 32-bit binary address that uniquely and universally defines the connection of a host or a router to the Internet. In Internet, no two devices can have the same IP For readability, we divide the IP address into 4 bytes. Dotted-decimal notation: Each byte is separated by dots. 8

5 Example Change the following IPv4 addresses from binary notation to dotted-decimal notation. Solution We replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add dots for separation. 9 Example Change the following IPv4 addresses from dotted-decimal notation to binary notation. Solution We replace each decimal number with its binary equivalent 10

6 Example Find the error, if any, in the following IPv4 addresses. Solution a. There must be no leading zero (045). b. There can be no more than four numbers. c. Each number needs to be less than or equal to 255. d. A mixture of binary notation and dotted-decimal notation is not allowed. 11 Note In classful addressing, the address space is divided into five classes: A, B, C, D, and E. 12

7 Classful addressing The address space is divided into five classes: A, B, C, D and E 13 Example Find the class of each address. a b c d Solution a. The first bit is 0. This is a class A address. b. The first 2 bits are 1; the third bit is 0. This is a class C address. c. The first byte is 14; the class is A. d. The first byte is 252; the class is E. 14

8 Number of blocks and block size in classful IPv4 addressing 15 Note In classful addressing, a large part of the available addresses were wasted. 16

9 Default masks for classful addressing 17 Note Classful addressing, which is almost obsolete, is replaced with classless addressing. 18

10 Example The figure below shows a block of addresses, in both binary and dotteddecimal notation, granted to a small business that needs 16 addresses. We can see that the restrictions are applied to this block. The addresses are contiguous. The number of addresses is a power of 2 (16 = 2 4 ), and the first address is divisible by 16. The first address, when converted to a decimal number, is 3,440,387,360, which when divided by 16 results in 215,024, Note In IPv4 addressing, a block of addresses can be defined as x.y.z.t /n in which x.y.z.t defines one of the addresses and the /n defines the mask. 20

11 Note The first address in the block can be found by setting the rightmost 32 n bits to 0s. 21 Example A block of addresses is granted to a small organization. We know that one of the addresses is /28. What is the first address in the block? Solution The binary representation of the given address is If we set rightmost bits to 0, we get or This is actually the block shown in figure below. 22

12 Note The last address in the block can be found by setting the rightmost 32 n bits to 1s. 23 Example Find the last address for the block in previous example. Solution The binary representation of the given address is If we set rightmost bits to 1, we get or This is actually the block shown in figure below. 24

13 Note The number of addresses in the block can be found by using the formula 2 32 n. 25 Example Find the number of addresses in previous example. Solution The value of n is 28, which means that number of addresses is or

14 Example Another way to find the first address, the last address, and the number of addresses is to represent the mask as a 32-bit binary (or 8-digit hexadecimal) number. This is particularly useful when we are writing a program to find these pieces of information. In Example 19.5 the /28 can be represented as (twenty-eight 1s and four 0s). Find: a. The first address b. The last address c. The number of addresses. 27 Example (continued) Solution a. The first address can be found by ANDing the given addresses with the mask. ANDing here is done bit by bit. The result of ANDing 2 bits is 1 if both bits are 1s; the result is 0 otherwise. 28

15 Example 19.9 (continued) b. The last address can be found by ORing the given addresses with the complement of the mask. ORing here is done bit by bit. The result of ORing 2 bits is 0 if both bits are 0s; the result is 1 otherwise. The complement of a number is found by changing each 1 to 0 and each 0 to 1. c. The number of addresses can be found by complementing the mask, interpreting it as a decimal number, and adding 1 to it. 29 A network configuration for the block /28 30

16 Network address Network address is an address that defines the network itself; it cannot be assigned to a host. All hostid bytes are 0s Defines the network to the rest of the Internet. First address in the block Given the network address, we can find the class of the address. Note The first address in a block is normally not assigned to any device; it is used as the network address that represents the organization to the rest of the world. 31 Levels of hierarchy Levels of Hierarchy To reach a host on the Internet, we must first reach the network by using the first portion of the address (netid) Then we must reach the host itself by using the second portion (hostid) IP addresses are designed with two levels of hierarchy. 32

17 Note Each address in the block can be considered as a two-level hierarchical structure: the leftmost n bits (prefix) define the network; the rightmost 32 n bits define the host. 33 Netid, Hostid Netid: Network address. Hostid: Node address 34

18 Hierarchy in telephone numbers 35 Three-level hierarchy in an IPv4 address Adding subnetworks creates an intermediate level of hierarchy in the IP addressing system. Now we have three levels: site, subnet, and host. The site is the first level. The second level is the subnet. The host is the third level. 36

19 Subnetting Sub-netting We can divide a network into sub-networks while making the world knows only the main network. In sub-netting, a network is divided into several smaller groups with each sub-network (or subnet) having its own sub-network address. 37 Example An ISP is granted a block of addresses starting with /16 (65,536 addresses). The ISP needs to distribute these addresses to three groups of customers as follows: a. The first group has 64 customers; each needs 256 addresses. b. The second group has 128 customers; each needs 128 addresses. c. The third group has 128 customers; each needs 64 addresses. Design the subblocks and find out how many addresses are still available after these allocations. 38

20 Example (continued) Solution Figure in slide 42 shows the situation. Group 1 For this group, each customer needs 256 addresses. This means that 8 (log2 256) bits are needed to define each host. The prefix length is then 32 8 = 24. The addresses are: 39 Example (continued) Group 2 For this group, each customer needs 128 addresses. This means that 7 (log2 128) bits are needed to define each host. The prefix length is then 32 7 = 25. The addresses are 40

21 Example (continued) Group 3 For this group, each customer needs 64 addresses. This means that 6 (log 2 64) bits are needed to each host. The prefix length is then 32 6 = 26. The addresses are: Number of granted addresses to the ISP: 65,536 Number of allocated addresses by the ISP: 40,960 Number of available addresses: 24, An example of address allocation and distribution by an ISP 42

22 Mask A router routes the packet based on network address and subnetwork address. A router inside a network routes based on subnetwork address but a router outside a network routes based on network address. Router uses the 32-bit mask to identify the network address. Routers outside an organization use a default mask; the routers inside an organization use a subnet mask Default mask 32-bit binary number that gives the network address when ANDed with an address in the block. 43 Default masks Class In Binary In Dotted-Decimal Using Slash A /8 B /16 C /24 Netid is retained and hostid sets to 0s. 44

23 Example A router outside the organization receives a packet with destination address Show how it finds the network address to route the packet. Solution: The router follows three steps: 1. The router looks at the first byte of the address to find the class. It is class B. 2. The default mask for class B is The router ANDs this mask with the address to get The router looks in its routing table to find out how to route the packet to this destination. Later, we will see what happens if this destination does not exist. 45 Subnet mask Number of 1s in a subnet mask is more than the number of 1s in the corresponding default mask. In a subnet mask, we change some of the leftmost 0s in the default mask to make a subnet mask. 46

24 Example A router inside the organization receives the same packet with destination address Show how it finds the subnetwork address to route the packet. Solution: The router follows three steps: 1. The router must know the mask. We assume it is /19, as shown in Figure The router applies the mask to the address, The subnet address is The router looks in its routing table to find how to route the packet to this destination. Later, we will see what happens if this destination does not exist. 47 Supernetting Although class A and B addresses are almost depleted, class C addresses are still available. In supernetting, an organization can combine several class C blocks to create a larger range of addresses. Several networks are combined to create a supernetwork. 48

25 Classless Addressing A range of addresses meant a block of addresses in class A, B, or C. What about a small business that needed only 16 addresses? Or a household that needed only two addresses? ISPs provide IP; people connect via dial-up modem, DSL, or cable modem to the ISP. Variable-length blocks: No class boundaries. Mask: Provide a block, it is given the first address and mask. Subnetting Classless InterDomain Routing (CIDR) 49 Dynamic Address Configuration Each computer has IP address, subnet mask, IP address of a router, IP address of a name server; This information is usually stored in a configuration file and accessed by the computer during the bootstrap (boot) process. Dynamic Host Configuration Protocol (DHCP) is a protocol designed to provide the information dynamically (based on demand). DHCP is a client-server program. When a DHCP client requests a temporary IP address, the DHCP server goes to the pool of available (unused) IP addresses and assigns an IP address for a negotiable period of time. When a DHCP client sends a request to a DHCP server, the server first checks its static database. If an entry with the requested physical address exists in the static database, the permanent IP address of the client is returned. On the other hand, if the entry does not exist in the static database, the server selects an IP address from the available pool, assigns the address to the client, and adds the entry to the dynamic database. 50

26 Addresses for private networks 51 A Network Address Translation (NAT) implementation NAT enables a user to have a large set of addresses internally and one address, or a small set of addresses, externally. The traffic inside can use the large set; the traffic outside, the small set. 52

27 Address Translation All the outgoing packets go through the NAT router, which replaces the source address in the packet with the global NAT address. All incoming packets also pass through the NAT router, which replaces the destination address in the packet (the NAT router global address) with the appropriate private address. 53 NAT address translation Using one IP address: private address to external address mapping. Limitation is that only the private network can initiate a connection and not vice-versa. Only one request at a time. 54

28 Five-column translation table Using a pool of IP addresses More than one global address is there and we map to one of them. Limited by the number of global IP. Using both IP and port numbers Mapping with IPs and Port numbers. 55 An ISP and NAT 56

29 14.2 IPv6 ADDRESSES Despite all short-term term solutions, address depletion is still a long-term problem for the Internet. This and other problems in the IP protocol itself have been the motivation for IPv6. Topics discussed in this section: Structure Address Space 57 Note An IPv6 address is 128 bits long. 58

30 IPv6 address in binary and hexadecimal colon notation 59 Abbreviated IPv6 addresses 60

31 Example Expand the address 0:15::1:12:1213 to its original. Solution We first need to align the left side of the double colon to the left of the original pattern and the right side of the double colon to the right of the original pattern to find how many 0s we need to replace the double colon. This means that the original address is. 61 Type prefixes for IPv6 addresses 62

32 Type prefixes for IPv6 addresses (continued) 63 Prefixes for provider-based unicast address 64

33 Multicast address in IPv6 65 Reserved addresses in IPv6 66

34 Local addresses in IPv SUMMARY (part 14) There are two popular approaches to packet switching: the datagram approach and the virtual circuit approach. In the datagram approach, each packet is treated independently of all other packets. At the network layer, a global addressing system that uniquely identifies every host and router is necessary for delivery of a packet from network to network. The Internet address (or IP address) is 32 bits (for IPv4) that uniquely and universally defines a host or router on the internet. The portion of the IP address that identifies the network is called the netid. The portion of the IP address that identifies the host or router on the network is called the hostid. There are five classes of IP addresses. Classes A, B, and C differ in the number of hosts allowed per network. Class D is for multicasting, and class E is reserved. The class of a network is easily determined by examination of the first byte. Unicast communication is one source sending a packet to one destination. Multicast communication is one source sending a packet to multiple destinations. Subetting divides one large network into several smaller ones. Subnetting adds an intermediate level of hierarchy in IP addressing. Default masking is a process that extracts the network address from an IP address. Subnet masking is a process that extracts the subnetwork address from an IP address Supernetting combines several networks into one large one. 68

35 14.3 SUMMARY continued (part 14) In classless addressing, there are variable-length blocks that belong to no class. The entire address space is divided into blocks based on organization needs. The first address and the mask in classless addressing can define the whole block. A mask can be expressed in slash notation which is a slash followed by the number of 1s in the mask. Every computer attached to the Internet must know its IP address, the IP address of a router, the IP address of a name server, and its subnet mask (if it is part of a subnet). DHCP is a dynamic configuration protocol with two databases. The DHCP server issues a lease for an IP address to a client for a specific period of time. Network address translation (NAT) allows a private network to use a set of private addresses for internal communication and a set of global Internet addresses for external communication. NAT uses translation tables to route messages. The IP protocol is a connectionless protocol. Every packet is independent and has no relationship to any other packet. Every host or router has a routing table to route IP packets. In next-hop routing, instead of a complete list of the stops the packet must make, only the address of the next hop is listed in the routing table. In network-specific routing, all hosts on a network share one entry in the routing table. In host-specific routing, the full IP address of a host is given in the routing table. In default routing, a router is assigned to receive all packets with no match in the routing table. A static routing table's entries are updated manually by an administrator. Classless addressing requires hierarchial and geographic routing to prevent immense routing tables. 69 References B.A. Forouzan, Data Communications and Networking, 4th edition, McGraw-Hill, 2007 W. Stalling, Local and Metropolitan Area Networks, 6 th edition, Prentice Hall, 2000 W. Stallings, Data and Computer Communications, 7 th edition, Prentice Hall, 2004 F. Halsall, Data Communications, Computer Networks and Open Systems, 4 th edition, Addison Wesley,