Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 272

Table 6-3 summarizes the IP address classes.

IP Address Classes

Table 6-3

Address Class High Order Bits Network Numbers

A 0xxxxxxx 1.0.0.0 to 126.0.0.0

B 10xxxxxx 128.0.0.0 to 191.255.0.0

C 110xxxxx 192.0.0.0 to 223.255.255.0

D 1110xxxx 224.0.0.1 to 239.255.255.255

E 1111xxxx 240.0.0.0 to 254.255.255.255

Private Address Space
Some network numbers are reserved for private use. These numbers are not routed on the
Internet. Private addresses are explained in RFC 1918, “Address Allocation for Private Internets.”
Published in 1996, private addresses were one of the first issues addressed with the concern that
globally unique address space would become exhausted. Many enterprises use private address
space in networks today, combined with accessing the Internet using NAT, which NAT is
covered later in this chapter.

The IP address space reserved for private Internets are 10.0.0.0, and 172.16.0.0 to 172.31.0.0,
and 192.168.0.0 to 192.168.225.0. This includes 1 Class A network, 16 Class B networks, and
256 Class C networks, respectively. Table 6-4 summarizes private address space.

Private Address Space Table 6-4

Class Type Start Network End Network

Class A 10.0.0.0/8 -

Class B 172.16.0.0/16 172.31.0.0/16

Class C 192.168.0.0/24 192.168.255.0/24

IP Address Subnets
Subnetting plays an important part in IP addressing. The subnet mask helps determine the
network, subnetwork, and host part of an IP address. The network architect uses subnetting to
manipulate the default mask to create subnetworks for LAN and WAN segments. As mentioned
earlier, one router interface with 16 million hosts off that interface is not a good thing.

Subnet masks are for Class A, B, and C addresses only. Multicast addresses do not use subnet
masks. Subnet masks are a 32-bit number where bits are set to 1 to establish the network portion
of the address, and a 0 is the host part of the address. The mask's bits set to 1 are contiguous