IP Addressing And The Routing Process

Transcription

1 The Ultimate CCNA Study Package - ICND 1 Chris Bryant, CCIE # Back To Index IP Addressing And The Routing Process Introduction To IP Addressing IP Addressing and Binary Conversions IP Address Classes Private IP Address Classes Introduction To The Routing Process Routing To Directly Connected Networks Routing To Non-Connected Networks Keeping Subnets On One "Side" Of The Router Introduction To Layer 3 Addresses For one host to successfully send data to another host, the sending host needs two addresses for the destination: destination MAC address destination IP address

2 In this section, we're going to concentrate on Internet Protocol (IP) addressing. IP addresses are often referred to as "Network addresses" or "Layer 3 addresses", since that is the OSI layer at which these addresses are used. The IP address format you're familiar with - addresses such as " " - are IP version 4 addresses. That address type is the focus of this section. IP version 6 addresses are now in use as well, and they're radically different from IPv4 addresses. IPv6 is a CCNA and CCNP topic, but is not covered on the CCENT exam. Even if you do not go on to the CCNA (and why wouldn't you?), you should become familiar with IPv6 addressing in your future networking studies. IPv6 looks really intimidating at first, but it's not that bad once you get over the initial shock of 128-bit addresses, as compared to IPv4's 32-bit addressing. The routing process and IP both operate at the Network layer of the OSI model, and the routing process uses IP addresses to move packets across the network in the most effective manner possible. In this section, we're going to first take a look at IP addresses in general, and then examine how routers make a decision on how to get data from source to destination. The routing examples in this section are simple ones, but they illustrate important fundamentals that you must have a firm grasp on before moving on to more complex examples. To do any routing, we've got to understand IP addressing, so let's start there! IP Addressing And An Introduction To Binary Conversions If you've worked as a network admin for any length of time, you're already familiar with IP addresses. Every PC on a network will have one, as will other devices such as printers. The term for a network device with an IP address is host, and I'll try to use that term as often as possible to get you used to it! The PC...err, the host I'm creating this document on has an IP address, shown here with the Microsoft command ipconfig. C:\>ipconfig Windows IP Configuration Ethernet adapter Local Area Connection: IP Address : Subnet Mask :

3 Default Gateway : We're going to discuss the default gateway setting later in this section, but for now let's concentrate on that IP address and the subnet mask. We're going to compare the IP address and subnet mask, because that will allow us to see what network this particular host belongs to. And to do so, we're going to convert both the IP address and the subnet mask to binary strings. Take a deep breath. It's not bad at all! We're going to look at binary conversions and subnetting in more detail in a future section, but this is a good place to introduce you to these conversions. And as you'll see later in this section, there's a very good reason I'm showing you how to do this now! First, we'll convert the IP address to a binary string. The format that we're used to seeing IP addresses take - like the shown here - is often called a dotted decimal address, since the values are in decimal and there are dots separating the decimals. (Clever, eh?) Each one of those numbers in the address - 192, 168, 1, and are decimal representations of a binary string, and a binary string is simply a string of ones and zeroes. Remember - "it's all ones and zeroes"! We'll convert the decimal 192 to binary first. All we need to do is use the following series of numbers and write the decimal that requires conversion on the left side: And all you have to do now is work from left to right and ask yourself one question: "Can I subtract this number from the current remainder?" Let's walk through this example and you'll see how easy it is! Looking at that chart, ask yourself "Can I subtract 128 from 192?" Certainly we can. That means we put a "1" under "128" Subtract 128 from 192 and the remainder is 64. so now we ask ourselves

4 "Can I subtract 64 from 64?" Certainly we can! Let's put a "1" under "64" Subtract 64 from 64, and you've got zero. You're practically done with your first binary conversion. Once you reach zero, just put a zero under every other remaining value, and you have your binary string! The resulting binary string for the decimal 192 is That's all there is to it! We're going to get plenty of practice with this skill before you take the CCENT and CCNA exams, but I'm sure you'll agree that it's a pretty simple process and nothing to be anxious about. If you know the basics of binary and decimal conversions, AND practice these skills diligently, you can answer any subnetting question Cisco asks you. But that's a topic for another chapter! I'll go ahead and show you the entire binary string for the IP address below, and the subnet mask is expressed in binary directly below it = = The subnet mask indicates where the network bits and host bits are. The network bits of the IP address are indicated by a "1" in the subnet mask, and the host bits are where the subnet mask has a "0". This address has 24 network bits, and the network portion of this address is in decimal. Here's why I showed you this conversion now - any IP addresses that have the exact same network portion are considered to be on the same subnet. If the network is configured correctly, hosts on the same subnet should be found on one "side" of the router, as shown below.

5 Assuming a subnet mask of for all hosts, we have two separate subnets x and x. Hosts on the same subnet are not separated by the router. What you don't want is the following: This could lead to a problem, since hosts in the same subnet are separated by a router. We'll see why this could be a problem when we examine the routing process later in this section, but for now keep in mind that having hosts in the same subnet separated by a router is not a good idea! The IP Address Classes Way back in the ancient times of technology - September 1981, to be exact - IP address classes were defined in RFC 791. If you'd like to read the actual RFC, you can find it quickly using that number and your favorite search engine. RFCs are Requests For Comments, which are technical proposals and/or documentation. Not always exciting, but it's well worth reading the RFC

6 that deals with the subject you're studying. Advanced technical exams occasionally like to ask about RFC numbers for a particular protocol or network service. To earn your CCENT and CCNA certifications, you've got to know these address classes and be able to identify what class an IP address belongs to. Here are the three ranges of addresses that can be assigned to hosts: Class A: Class B: Class C: The following classes are reserved and cannot be assigned to hosts: Class D: Reserved for multicasting, a topic not covered on the CCENT or CCNA exams. Class E: Reserved for future use, also called "experimental addresses". Any address with a first octet of 127 is reserved for loopback interfaces. This range is *not* for Cisco router loopback interfaces, though. For your exams, I would strongly recommend that you know which ranges can be assigned to hosts and which ones cannot. Also, be able to identify which class a given IP address belongs to. It's straightforward, but I can practically guarantee those skills will serve you well on exam day! The rest of this section concentrates on Class A, B, and C networks. Each class has its own default network mask, default number of network bits, and default number of host bits. We'll manipulate these bits just a bit in the subnetting section, but you must know the following values in order to answer subnetting questions successfully - in the exam room or on the job! Class A: Default network mask: Default number of network bits: 8 Default number of host bits: 24 Class B: Default network mask: Default number of network bits: 16 Default number of host bits: 16 Class C: Default network mask:

7 Default number of network bits: 24 Default number of host bits: 8 We worked just a bit with the network and host bits earlier in this section, and we're going to work with them a lot more in the subnetting sections later in the course. For now, make sure that you know... How to identify the class of an IP address Which addresses can be assigned to hosts (Class A, B, C) Which addresses cannot be assigned to hosts (Class D, E, any address beginning with 127) The default network mask, network bits, and host bits for Class A, B, and C addresses I'll remind you of the default masks, network bits, and host bits when we get to subnetting! The RFC 1918 Private Address Classes If you've worked on different networks in your IT career, you may have noticed that the hosts at different sites use similar IP addresses. That's because certain IP address ranges have been reserved for internal networks - that is, networks with hosts that do not need to communicate with other hosts outside their own little internal network, and that do not need to access the internet. Address classes A, B, and C all have their own reserved range of addresses. You should be able to recognize an address from any of these ranges immediately. Class A: Class B: Class C: You should be ready to identify those ranges in that format, or with the dotted decimal masks, or with prefix notation. More about prefix notation later in this section. Class A: , or /8 Class B: , or /12 Class C: , or /16 You may already be thinking "Hey, we use some of those addresses on our network hosts and they get out to the Internet with no problem at all." The network services NAT and PAT (Network Address Translation and Port Address Translation) make that possible, but these are not default behaviors. We have to configure NAT and PAT manually. We're going to do just that later in this course, but for now, make sure you know those

8 three address ranges cold! Introduction To The Routing Process You've probably heard the term "routing protocol", and we're going to look at one popular routing protocol before the end of this course. Before we start working with protocols, we need to understand the very basics of the routing process and how routers decide where to send packets. Remember, we're at Layer 3 of the OSI model here, so the data is in the form of packets. First we'll take a look at a very basic setup and follow the decision-making process from the point of view of the host, then the router. We'll then examine the previous example in this section to see why it's a bad idea to have hosts from the same subnet separated by a router. Let's take another look at a PC's ipconfig output. C:\>ipconfig Windows IP Configuration Ethernet adapter Local Area Connection: IP Address : Subnet Mask : Default Gateway : When this host is ready to send data, there are two and only two possibilities: The destination IP address is on the network It's on another network If the destination is on the same subnet as the host, the packet's destination IP address will be that of the destination host. In the following example, this PC is sending packets to , a host on the same subnet, so there is no need for the router to get involved. In effect, those packets go straight to

9 If wants to send packets to the host at , the sending host knows it's not on the same subnet as In that case, the host will send the packets to its default gateway - in this case, the router's ethernet0 interface. The host is basically saying "I have no idea where this address is, so I'll send it to my default gateway and let that device figure it out." When a router receives a packet, there are three possibilities regarding its destination: Destined for a directly connected network

10 Destined for a non-directly connected network that the router has an entry for in its routing table Destined for a non-directly connected network that the router does not have an entry for How A Router Handles A Packet Destined For A Directly Connected Network We'll use the following network in this section: The router has two Ethernet interfaces, referred to in the rest of this example as "E0" and "E1". The switch ports will not have IP addresses, but the router's Ethernet interfaces will and they are shown in the exhibit. Host A sends a packet destined for Host B at The router will receive that packet on its E0 interface and see the destination IP address of The router will then check its routing table to see if there's an entry for the network. Assuming no static routes or dynamic routing protocols have been configured, the router's IP routing table will look like this:

11 R1#show ip route Codes: C - connected, S - static Gateway of last resort is not set C C /8 is directly connected, Ethernet /8 is directly connected, Ethernet0 See the "C" and the "S" next to the word "codes"? You'll usually see anywhere from different types of routes there, and I've removed those for clarity's sake. You'll see the full table later in the course. Also note that you don't see the mask expressed as " " - you see it as "/8" instead. This is called prefix notation, and the number simply represents the number of 1s at the beginning of the network mask when it's expressed in binary. That "/8" is pronounced "slash eight" = binary string = /8 We'll get plenty of practice with prefix notation in the subnetting section! Also, earlier in this section I listed the prefix notation equivalent of the dotted decimal mask with the RFC 1918 private address ranges. Make sure you're ready to identify those ranges no matter if you're presented with dotted decimal masks or prefix notation. Now back to our network! The "C" indicates a directly connected network, and there is an entry for The router will then send the packet out its E1 interface and Host B will receive it. Simple enough, right? Of course, the destination network will not always be directly connected - we're not getting off that easy! How The Router Handles A Packet Destined For A Remote Network That Is Present - Or Not - In The Routing Table

12 Here's the topology for this example: If Host A wants to transmit packets to Host C, we've got an issue right off the bat. The packets will not get there if Router 1 ("R1") is not using some kind of static route or dynamic routing protocol - and neither of those is present by default. Once we apply those IP addresses and then open the interfaces, we'll see some connected routes, but that's it. When R1 receives the packet destined for , R1 will perform a routing table lookup to see if there's a route for The problem is that there is no such route, since R1 only knows about the directly

13 connected networks and R1#show ip route Codes: C - connected, S - static Gateway of last resort is not set C C /8 is directly connected, Ethernet /8 is directly connected, Ethernet0 Without some kind of route to , the packet will simply be dropped by R1. As you'll see later in the course, we can use a static route or a dynamic routing protocol to get that route - but hey, why wait? Here's what such a static route would look like, and remember that we'll spend a lot of time with that route type later in the course. Static routes are created with the ip route command. R1(config)#ip route ethernet1 The routing table now displays a route for the /8 network. The letter "S" indicates a static route. R1#show ip route Codes: C - connected, S - static C C S /8 is directly connected, Ethernet /8 is directly connected, Ethernet /8 is directly connected, Ethernet1 R1 now has an entry for the network, and sends the packet out its E1 interface. R2 will have no problem forwarding the packet destined for , since R2 is directly connected to that network.

14 If Host C wants to respond to Host A's packet, there would be a problem at R2, since the incoming destination address would be , and R2 has no entry for that network. A static route or dynamic routing protocol would be needed to get such a route into R2's routing table. Why We Want To Keep Hosts In One Subnet On One Side Of The Router Earlier in this section, the following topology served as an example of how not to configure a network.

15 Now that we've gone through some routing process examples, let's see why this is a bad idea. Let's say a packet destined for is coming in on another router interface. The router receives that packet and performs a routing table lookup for , and sees that network is directly connected - via interface E0. The router will then send the packet out the E0 interface, even though the destination IP address is actually found off the E1 interface!

16 In future studies, you'll learn ways where you could get the packets to but for your CCENT and CCNA exams, just keep in mind that it's a good practice to keep all members of a given subnet on one side of a router. It's good practice for production networks, too! Back To Index Copyright 2008 The Bryant Advantage. All Rights Reserved.