BGP

Chapter 2. IP Addressing and the BGP Protocol

This chapter provides an overview of the IP address architecture and some interdomain routing history, followed by an explanation of the BGP protocol, information on how BGP relates to routing in general, and a discussion of Multiprotocol BGP.

IP Addresses

IP addresses are made up of two parts: the network part and the host part. Because IP addresses are only 32 bits in length, it’s not possible to have both a large host part (to accommodate networks with many hosts) and a large network part (to accommodate a large number of networks) at the same time. To get around this, there are three classes of IP addresses:

• Class A addresses, with a 7-bit network part and a 24-bit host part, allow 128 networks with 16 million hosts each. The highest bit is always set to 0 in Class A address, so the first byte of Class A IP addresses ranges from 0 to 127.

• Class B addresses, with a 14-bit network part and a 16-bit host part, allow 16384 networks with 65534 hosts each. The two highest bits are always set to 10 in Class B addresses, so the first byte of Class B IP addresses ranges from 128 to 191.

• Class C addresses, with a 21-bit network part and an 8-bit host part, allow 2 million networks with 254 hosts each. The three highest bits are always set to 110 in Class C addresses so, the first byte of Class C IP addresses ranges from 192 to 223.

Note that the first address in a network (the all-zeros address) is the network address, and can’t be used. The last ...