User guide

3-12
Configuration Guide for Cisco DSLAMs with NI-2
OL-2074-03
Chapter 3 Initially Configuring the Cisco DSLAM
Configuring IP Interface Parameters
Defining an IP address
This section provides a summary of IP addressing concepts for those who are familiar with
IP addressing.
Internet addresses are 32-bit values assigned to hosts that use the IP protocols. These addresses are in
dotted decimal format (four decimal numbers separated by periods), such as 192.17.5.100. Each number
is an 8-bit value between 0 and 255.
IP addresses are divided into three classes. These classes differ in the number of bits allocated to the
network and host portions of the address:
• The Class A Internet address format allocates the highest 8 bits to the network field and sets the
highest-order bit to 0 (zero). The remaining 24 bits form the host field.
• The Class B Internet address allocates the highest 16 bits to the network field and sets the two
highest-order bits to 1, 0. The remaining 16 bits form the host field.
• The Class C Internet address allocates the highest 24 bits to the network field and sets the three
highest-order bits to 1, 1, 0. The remaining 8 bits form the host field.
The default IP address is none.
Enter your Internet address in dotted decimal format for each interface you plan to configure.
Defining Subnet Mask Bits
Subnetting is an extension of the Internet addressing scheme that allows multiple physical networks to
exist within a single Class A, B, or C network. The subnet mask determines whether subnetting is in
effect on a network. The usual practice is to use a few of the far-left bits in the host portion of the network
address to assign a subnet field.
Internet addressing conventions allow a total of 24 host bits for Class A addresses, 16 host bits for
Class B addresses, and 8 host bits for Class C addresses. When you are further subdividing your network
(that is, subnetting your network), the number of host addressing bits is divided between subnetting bits
and actual host address bits. You must specify a minimum of two host address bits, or the subnetwork
is not populated by hosts.
Note Because all zeros in the host field specifies the entire network, subnetting with subnet address 0 is illegal
and is strongly discouraged.
Table 3-3 provides a summary of subnetting parameters.
Table 3-3 Subnetting Parameters
First Class First Byte Network Bits Host Bits
Max Subnet Bits Min Address Bits
A 1 to 126 8 22 2
B 128 to 191 16 14 2