Configuring Ip Address

Configuring IP

The Internet Protocol (IP) is the only routed protocol that is turned on by default on a Cisco router running IOS. The acronym IP is actually an abbreviated way of writing TCP/IP (Transmission Control Protocol/Internet Protocol), which is the suite of protocols and applications used on the Internet and many private internetworks.

We are going to begin the coverage of IP configuration with a brief overview of IP and how IOS processes its traffic. Then we will delve into the configuration itself and make some modifications to the internetwork that we built in Chapter 3. The only version of IP covered in this book is IP version 4.

IP Addressing

All hosts that run IP must have a unique IP address. An IP address is a logical address that is independent of a host’s hardware. IP addressing is perhaps the most mystifying part of IP for people who are new to the networking world; however, it really is very simple. To understand, you just need to use a little bit of binary (base 2) arithmetic and decimal (base 10) arithmetic.

IP addresses are 32 bits long, and the normal way of writing them is called dotted-decimal notation. To write an address in dotted-decimal notation, we divide the 32 bits of the address into four 8-bit chunks. Each 8-bit chunk is called an octet or a byte. We then convert the octets from binary to decimal and put dots (.) between them. Figure 7-1 shows four forms of the same IP host address.

<<<J115 – Figure 7-1 IP Address Notations>>>

The first form in Figure 7-1 is normal binary, just a string of 32 ones and zeros; each one and zero is a bit. This form is difficult for people to read, but it is what a computer, like a router, sees. The decimal representation of the binary address has a rather large value; 32 bits can represent decimal numbers between 0 and 4,294,967,295. The second line is the decimal equivalent of the first line. How would you like to read a number like that every time you wanted to communicate a host address to someone?

The third line of Figure 7-1 is just an intermediate step toward the last line, with the 32 bits divided into four octets. The last form, dotted-decimal notation, is the one that we use, and it is the easiest to read and write.

Since each of the four quarters (octets) of an IP address is eight bits and the decimal values that can be represented with eight bits range from 0 to 255, the value of any one of the numbers in a dotted-decimal IP address cannot exceed 255.

IP addresses have two main parts – a network part that identifies the network where a host resides and a node part that identifies a specific host on the network (sort of like a street name and a house number). The network and node parts together make up the full IP address of a host. The network part is used by routing software to determine for which network a packet is destined. The node part is used by routing software to send a packet to an individual host once the packet has reached the host’s network. Just to make this a little more exciting, the line between network and node moves.

There are three things that can be used to tell which part of an address is network and which part of an address is node. These are as follows:

Network Mask

Prefix Length

Class

The network mask explicitly specifies which part of an IP address represents a network. The network mask is 32 bits long and is normally written in dotted-decimal notation just like an address. An IP address and mask are usually written together, with the mask immediately after the address.

A network mask has binary ones in the bit positions that represent the network part of an address and binary zeros in the bit positions that represent the node part of an address. The binary ones must start from the left (most-significant) side of the mask and extend contiguously (They must be side by side.) until the network part ends; the rest of the mask must be all zeros. For example, the mask

255.255.255.0

when paired with an IP address, would tell us that the first 24 bits of the address is considered network and the last eight bits are node. This becomes clear when the binary equivalent of this mask is examined as shown below:

11111111 11111111 11111111 00000000 (Binary)

255 . 255 . 255 . 0 (Dotted-Decimal)

We see that the mask has 24 ones, starting from the