Contents

Configuring IP addressing· 1

Overview·· 1

IP address classes· 1

Special IP addresses· 2

Subnetting and masking· 2

Assigning an IP address to an interface· 3

Configuration guidelines· 3

Configuration procedure· 4

Displaying and maintaining IP addressing· 4

Configuring IP addressing

The IP addresses in this chapter refer to IPv4 addresses unless otherwise specified.

This chapter describes IP addressing basics and manual IP address assignment for interfaces. Dynamic IP address assignment (BOOTP and DHCP) and PPP address negotiation are beyond the scope of this chapter.

Overview

This section describes the IP addressing basics.

IP addressing uses a 32-bit address to identify each host on an IPv4 network. To make addresses easier to read, they are written in dotted decimal notation, each address being four octets in length. For example, address 00001010000000010000000100000001 in binary is written as 10.1.1.1.

IP address classes

Each IP address breaks down into the following sections:

·     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, as shown in Figure 1. The shaded areas represent the address class. The first three classes are most commonly used.

Figure 1 IP address classes

Table 1 IP address classes and ranges

Special IP addresses

The following IP addresses are for special use and 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 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 Subnetting a Class B network

Subnetting increases the number of addresses that cannot be assigned to hosts. Therefore, using subnets means accommodating 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—65534 (2¹⁶ – 2) hosts. (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 nine bits of the host-id for subnetting provides 512 (2⁹) subnets. However, only seven bits remain available for the host ID. This allows 126 (2⁷ – 2) hosts in each subnet, a total of 64512 (512 × 126) hosts.

Assigning an IP address to an interface

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, DHCP, or PPP address negotiation. If you change the IP address assignment method, the new IP address will overwrite the previous address.

An interface can have one primary address and multiple secondary addresses.

Typically, you need to configure a primary IP address for an interface. If the interface connects to multiple subnets, configure primary and secondary IP addresses on the interface so the subnets can communicate with each other through the interface.

Configuration guidelines

Follow these guidelines when you assign an IP address to an interface:

·     An interface can have only one primary IP address. A newly configured primary IP address overwrites the previous one.

·     You cannot assign secondary IP addresses to an interface that obtains an IP address through BOOTP, DHCP, or PPP address negotiation.

·     The primary and secondary IP addresses assigned to the interface can be located on the same network segment. Different interfaces on your device must reside on different network segments.

Configuration procedure

To assign an IP address to an interface:

Displaying and maintaining IP addressing

Execute display commands in any view.