Earlier, you read that IPv4 addresses represent four eight-digit binary numbers. That means that each number could be 00000000 to 11111111 in binary, or 0 to 255 in decimal (base-10). In other words, 0.0.0.0 to 255.255.255.255. However, some numbers in that range are reserved for specific purposes on TCP/IP networks. These reservations are recognized by the authority on TCP/IP addressing, the Internet Assigned Numbers Authority (IANA). Four specific reservations include the following:
- 0.0.0.0 -- This represents the default network, which is the abstract concept of just being connected to a TCP/IP network.
- 255.255.255.255 -- This address is reserved for network broadcasts, or messages that should go to all computers on the network.
- 127.0.0.1 -- This is called the loopback address, meaning your computer's way of identifying itself, whether or not it has an assigned IP address.
- 169.254.0.1 to 169.254.255.254 -- This is the Automatic Private IP Addressing (APIPA) range of addresses assigned automatically when a computer's unsuccessful getting an address from a DHCP server.
The other IP address reservations are for subnet classes. A subnet is a smaller network of computers connected to a larger network through a router. The subnet can have its own address system so computers on the same subnet can communicate quickly without sending data across the larger network. A router on a TCP/IP network, including the Internet, is configured to recognize one or more subnets and route network traffic appropriately. The following are the IP addresses reserved for subnets:
- 10.0.0.0 to 10.255.255.255 -- This falls within the Class A address range of 220.127.116.11 to 127.0.0.0, in which the first bit is 0.
- 172.16.0.0 to 172.31.255.255 -- This falls within the Class B address range of 18.104.22.168 to 22.214.171.124, in which the first two bits are 10.
- 192.168.0.0 to 192.168.255.255 -- This falls within the Class C range of 192.0.0.0 through 126.96.36.199, in which the first three bits are 110.
- Multicast (formerly called Class D) -- The first four bits in the address are 1110, with addresses ranging from 188.8.131.52 to 184.108.40.206.
- Reserved for future/experimental use (formerly called Class E) -- addresses 240.0.0.0 to 254.255.255.254.
The first three (within Classes A, B and C) are those most used in creating subnets. Later, we'll see how a subnet uses these addresses. The IANA has outlined specific uses for multicast addresses within Internet Engineering Task Force (IETF) document RFC 5771. However, it hasn't designated a purpose or future plan for Class E addresses since it reserved the block in its 1989 document RFC 1112. Before IPv6, the Internet was filled with debate about whether the IANA should release Class E for general use.
Next, let's see how subnets work and find out who has those non-reserved IP addresses out on the Internet.