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• Padding: A variable-length field used to make the header of the datagram an integral multiple of 32-bit words.
6.1.2 IP Addresses
As we saw above, IP addresses are 32 bits long. An IP address is divided into two parts: a network and a suffix. The network identifies the physical network that the host computer is attached to. The suffix identifies the host computer itself. The size of these two fields vary according to the class of the IP address. Specifically, five different classes of addresses - A, B, C, D, and E - have been defined (see Figure 6.2).
0 ...... .......................
1 0 Network Suffix
1 1 0 Network Suffix
1 1 1 0 Multicast address
1 1 1 1 0 Reserved for future use
Figure 6.2 The IP address classes.
THE MULTI-PROTOCOL LABEL SWITCHING (MPLS) ARCHITECTURE
Classes A, B and C are called the primary classes because they are used for host addresses. Class D is used for multicasting; class E is reserved for future use. The first field determines the class of the IP address, and it ranges from 1 bit for a class A address to 5 bits for a class E addresses. The second field gives the network address, and the third field is the suffix which gives the host address.
In class A, there is a 7-bit network address and a 24-bit host address, resulting to 128 network addresses and 16,777,216 host addresses. In class B, there is a 14-bit network address and a 16-bit host address, resulting to 16,384 network addresses and 65,536 host addresses. In class C, there is a 21-bit network address and an 8-bit host address, resulting to 2,097,152 network addresses and 256 host addresses.
Network addresses are usually written in the dotted decimal notation. That is, each byte is written in decimal, ranging from 0 to 255. As an example, the IP address 00000111 00000010 00000000 00000010 will be written as 18.104.22.168. Using this notation, we have that the range of class A addresses is from 22.214.171.124 to 127.255.255.255, for class B we have a range of values from 126.96.36.199 to 188.8.131.52, and for class C we have a range of 192.0.0.0 to 184.108.40.206.
Class C is very common, whereas class A is rarely used since there are only few networks with that large number of hosts. IP reserves host address 0 to denote the address of a network. For instance, in the class B address 220.127.116.11 the network field is 128.32 and the suffix is 0.0. This indicates the address of the network 128.32. For broadcasting within the network, IP uses the address 18.104.22.168.
IP assigns multiple IP addresses to routers, since a router is attached to multiple networks. Specifically, a router has one IP address for each network that it is attached to. An individual host connected to multiple networks has also multiple IP addresses, one for each network connection. Such a host is referred to as multihomed.
The IP address structure described above introduces a two-level hierarchy. The first level is the network address and the second level is the host address carried in the suffix. In many cases, these two levels of addressing is not enough. For instance, if we consider an organization with a B class address, then all of the hosts appear to be organized into a single group, described by the network address. However, hosts within an organization are typically grouped together to form a number of different LANs. In order to distinguish the LANs the suffix of the IP address is subdivided into a subnet part and a host part. Each LAN is assigned a subnet address carried in the subnet part, and a host in the LAN is assigned an address which is carried in the host part. The actual parsing of the suffix in these two subfields is dictated by a subnet mask. The subnet mask is only known to the routers within the network since the subnets are not visible outside the network. This technique is known as subnetting.
Classless inter-domain routing (CIDR)
In the early 90s, it became apparent that the rapid expansion of the Internet would cause a depletion of IP addresses and an explosion of the routing tables. The main cause for address depletion was the wasteful usage of class B addresses; many organizations used a class B address, but only had a small number of hosts, thus leaving the host address
THE INTERNET PROTOCOL (IP): A PRIMER
space largely unused. The routing table explosion was due to the fact that a router keeps all of the addresses of all of the registered networks.
In order to alleviate these two problems the classless inter-domain routing (CIDR) scheme was proposed. This scheme permits the assignment of contiguous class C addresses and at the same time it reduces the number of entries required in a routing table.