IP addressing configuration· 1

IP addressing overview·· 1

IP address classes 1

Special IP addresses 2

Subnetting and masking· 2

Configuring IP addresses 3

Assigning an IP address to an interface· 3

Displaying and maintaining IP addressing· 3

This chapter includes these sections:

·          IP addressing overview

·          Configuring IP addresses

·          Displaying and maintaining IP addressing

IP addressing overview

IP address classes

IP addressing uses a 32-bit address to identify each host on a network. To make addresses easier to read, they are written in dotted decimal notation, each address being four octets in length. For example, address 00001000000000010000000100000001 in binary is written as 10.1.1.1.

Each IP address breaks down into two parts:

·          Net ID: Identifies a network. The first several bits of a net ID, known as the class field or class bits, identify the class of the IP address.

·          Host ID: Identifies a host on a network.

IP addresses are divided into five classes, shown in Figure 1. The shaded areas represent the address class. The first three classes are widely used.

Figure 1 IP address classes

Table 1 describes the address ranges of these five classes.

Table 1 IP address classes and ranges

Special IP addresses

The following IP addresses are for special use, and they cannot be used as host IP addresses.

·          IP address with an all-zero net ID: Identifies a host on the local network. For example, IP address 0.0.0.16 indicates the host with a host ID of 16 on the local network.

·          IP address with an all-zero host ID: Identifies a network.

·          IP address with an all-one host ID: Identifies a directed broadcast address. For example, a packet with the destination address of 192.168.1.255 will be broadcast to all the hosts on the network 192.168.1.0.

Subnetting and masking

Subnetting divides a network down into smaller networks called subnets by using some bits of the host ID to create a subnet ID.

Masking identifies the boundary between the host ID and the combination of net ID and subnet ID.

Each subnet mask comprises 32 bits that correspond to the bits in an IP address. In a subnet mask, consecutive ones represent the net ID and subnet ID, and consecutive zeros represent the host ID.

Before being subnetted, Class A, B, and C networks use these default masks (also called natural masks): 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively.

Figure 2 shows how a Class B network is subnetted.

Figure 2 Subnet a Class B network

Subnetting increases the number of addresses that cannot be assigned to hosts. Therefore, using subnets means accommodating somewhat fewer hosts

For example, a Class B network without subnetting can accommodate 1022 more hosts than the same network subnetted into 512 subnets.

·          Without subnetting: 65,534 hosts (2¹⁶ – 2). (The two deducted addresses are the broadcast address, which has an all-one host ID, and the network address, which has an all-zero host ID.)

·          With subnetting: Using the first 9 bits of the host-id for subnetting provides 512 (2⁹) subnets. However, only 7 bits remain available for the host ID. This allows 126 (2⁷ – 2) hosts in each subnet, a total of 64,512 hosts (512 × 126). 

Configuring IP addresses

An interface must have an IP address to communicate with other hosts. You can either manually assign an IP address to an interface, or configure the interface to obtain an IP address through BOOTP or DHCP. If you change the way an interface obtains an IP address, the new IP address will overwrite the previous one.

Assigning an IP address to an interface

Follow these steps to assign an IP address to an interface:

Displaying and maintaining IP addressing