Internet Routing Architectures, Second Edition-CISCO Press
The addressing scheme that is used today in the Internet is based on version 4 of the Internet
Protocol (IPv4)[], usually referred to simply as IP. This section discusses the following:
• Basic IP addressing
• Basic IP subnetting
• Variable-length subnet mask (VLSM
Basic IP Addressing
An IP address is a unique 4-octet (32-bit) value expressed in dotted-decimal (or dotted-quad)notation of the form W.X.Y.Z, where periods (dots) are used to separate each of the 4 octets of the address (for example, 10.0.0.1). The 32-bit address field consists of two parts: a network or link number (which represents the network portion of the address) and a host number (which identifies a host on the network segment).
The network and host boundaries were traditionally defined based on the class of the IP address, with five defined classes (three of which are used for unicast addressing): A, B, C, D,and E.
Table 1-1 illustrates the different classes of address space and their functions.
Class Address Range High-Order Bits Network Bits Host Bits Function
A 0.0.0.0 to 127.255.255.255 0 7 24 Unicast
B 128.0.0.0 to 191.255.255.255 10 14 16 Unicast
C 192.0.0.0 to 223.255.255.255 110 21 8 Unicast
D 224.0.0.0 to 239.255.255.255 1110 Multicast
E 240.0.0.0 to 255.255.255.255 1111 Reserved
Notice that only Class A, B, and C addresses are used for unicast. Class D addresses are used for multicast, and Class E address space is reserved. Several addresses within these classes are reserved for special use. Table 3-2 lists some of these addresses.
Table 1-2, Special-Purpose IP Addresses
Address Range Purpose
0.0.0.0 Unknown network; commonly represents default
10.0.0.0 – 10.255.255.255 Reserved for private use (RFC 1918)
127.0.0.0 – 127.255.255.255 Reserved for loopback/local address
172.16.0.0 – 172.31.255.255 Reserved for private use (RFC 1918)
192.168.0.0 – 192.168.255.255 Reserved for private use (RFC 1918)
255.255.255.255 Limited broadcast
This class-based addressing scheme is often referred to as the classful model. The different classes lend themselves to different network configurations, depending on the desired ratio of networks to hosts. The full implications of the different classes will become more apparent as this chapter proceeds. The next few sections focus on the basic definitions of each class.
Class A Addressing
Class A networks are represented by a 0 in the leftmost bit position of the address. The first octet (bits 0 to 7) of the address, beginning from the leftmost bit, represents the network number, and the remaining 3 octets (bits 8 to 31) represent a host number on that network. An example of a Class A network is 124.0.0.1, where 124.0.0.0 represents the network number and the host number is 1. The outcome of this representation, illustrated in Figure-1, is 128(27) Class A network numbers. However, because 0.0.0.0 is not a valid network number, only 127(27–1) Class A addresses are possible.
Figure-1 General Class A Address Format
A 0.0.0.0 to 127.255.255.255 0 7 24 Unicast
B 128.0.0.0 to 191.255.255.255 10 14 16 Unicast
C 192.0.0.0 to 223.255.255.255 110 21 8 Unicast
D 224.0.0.0 to 239.255.255.255 1110 Multicast
E 240.0.0.0 to 255.255.255.255 1111 Reserved
Notice that only Class A, B, and C addresses are used for unicast. Class D addresses are used for multicast, and Class E address space is reserved. Several addresses within these classes are reserved for special use. Table 3-2 lists some of these addresses.
Table 1-2, Special-Purpose IP Addresses
Address Range Purpose
0.0.0.0 Unknown network; commonly represents default
10.0.0.0 – 10.255.255.255 Reserved for private use (RFC 1918)
127.0.0.0 – 127.255.255.255 Reserved for loopback/local address
172.16.0.0 – 172.31.255.255 Reserved for private use (RFC 1918)
192.168.0.0 – 192.168.255.255 Reserved for private use (RFC 1918)
255.255.255.255 Limited broadcast
This class-based addressing scheme is often referred to as the classful model. The different classes lend themselves to different network configurations, depending on the desired ratio of networks to hosts. The full implications of the different classes will become more apparent as this chapter proceeds. The next few sections focus on the basic definitions of each class.
Class A Addressing
Class A networks are represented by a 0 in the leftmost bit position of the address. The first octet (bits 0 to 7) of the address, beginning from the leftmost bit, represents the network number, and the remaining 3 octets (bits 8 to 31) represent a host number on that network. An example of a Class A network is 124.0.0.1, where 124.0.0.0 represents the network number and the host number is 1. The outcome of this representation, illustrated in Figure-1, is 128(27) Class A network numbers. However, because 0.0.0.0 is not a valid network number, only 127(27–1) Class A addresses are possible.
Figure-1 General Class A Address Format
After the network is defined, the first and last host addresses within the network serve special functions. The first address (124.0.0.0 in the previous example) is used to represent the network number, and the last address of the network is used to represent the directed broadcast address of the network (124.255.255.255). Therefore, Class A addresses have only 16,777,214 (224–2) hosts per network, rather than 16,777,216 (224) hosts per network.
Class B Addressing
Class B networks are represented by a 1 and a 0 in the leftmost two bits of the address. The first two octets of the address (bits 0 to 15) represent the network portion of the address, and the remaining two octets (bits 16 to 31) represent the host number of that network. The outcome of this representation, illustrated in Figure-2, is 16,384 (214) network numbers, with 65,534 (216–2) hosts per network. An example of a Class B address is 172.16.0.1, where 172.16.0.0 is the Class B network and 1 is the host.
Figure-2 General Class B Address Format
Class C Addressing
Class C networks are represented by 1, 1, and 0 in the leftmost three bits of the address. The first three octets (bits 0 to 23) represent the network number, and the last octet (bits 24 to 31) represents the host number in that network. The outcome of this representation, as illustrated in Figure-3, is 2,097,152 (221) network numbers with 254 (28–2) hosts per network. An example of a Class C network is 192.11.1.1, where 192.11.1.0 is the network number and the host number is 1.
Figure-3. General Class C Address Format
Class D Addressing
Class D networks are represented by 1, 1, 1, and 0 in the leftmost 4 bits of the address. The Class D address space is reserved for multicast, used to represent multicast group numbers.
Class E Addressing
Class E networks are represented by 1, 1, 1, and 1 in the leftmost 4 bits of the address. Class E address space is currently reserved for experimental use.
Class B Addressing
Class B networks are represented by a 1 and a 0 in the leftmost two bits of the address. The first two octets of the address (bits 0 to 15) represent the network portion of the address, and the remaining two octets (bits 16 to 31) represent the host number of that network. The outcome of this representation, illustrated in Figure-2, is 16,384 (214) network numbers, with 65,534 (216–2) hosts per network. An example of a Class B address is 172.16.0.1, where 172.16.0.0 is the Class B network and 1 is the host.
Figure-2 General Class B Address Format
Class C Addressing
Class C networks are represented by 1, 1, and 0 in the leftmost three bits of the address. The first three octets (bits 0 to 23) represent the network number, and the last octet (bits 24 to 31) represents the host number in that network. The outcome of this representation, as illustrated in Figure-3, is 2,097,152 (221) network numbers with 254 (28–2) hosts per network. An example of a Class C network is 192.11.1.1, where 192.11.1.0 is the network number and the host number is 1.
Figure-3. General Class C Address Format
Class D Addressing
Class D networks are represented by 1, 1, 1, and 0 in the leftmost 4 bits of the address. The Class D address space is reserved for multicast, used to represent multicast group numbers.
Class E Addressing
Class E networks are represented by 1, 1, 1, and 1 in the leftmost 4 bits of the address. Class E address space is currently reserved for experimental use.