Linux in a Nutshell, Fifth Edition

Overview of TCP/IP | 23

Sys Admin

Overview

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

A standard called Classless Inter-Domain Routing (CIDR, or supernetting)

extends the class system’s idea of using initial bits to identify where packets

should be routed. Under CIDR, a new domain can be created with any number of

fixed leftmost bits (not just a multiple of eight). A CIDR address looks like a

regular IPv4 address followed by a slash and a number indicating the number of

network bits. For example: 192.168.32.1/24 or 128.66.128.1/17. Virtually all

Internet gateway hosts now use CIDR.

IPv6, a newer standard, changes the method of addressing and increases the

number of fields. An IPv6 address is 128 bits. Part of an IPv6 address is based on

the Media Access Control (MAC) address of the network interface. The MAC

address is unique for each network interface. When written, it is usually divided

into eight 16-bit hexadecimal blocks separated by colons. For example:

FE80:0000:0000:0000:0202:B3FF:FE1E:8329

To shorten this, leading zeros may be skipped, and any one set of consecutive

zeros can be replaced with double colons. For example, the above address can be

reduced to:

FE80::202:B3FF:FE1E:8329

When IPv4 and IPv6 networks are mixed, the IPv4 address can be packed into the

lower four bytes, yielding an address like 0:0:0:0:0:0:192.168.1.2, ...