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There are several IP addresses that are special in one way or another. These addresses are for special purposes or are to be put to special use.

• Addresses significant to every IP subnet
  • Network Address
  • Broadcast Address
• Addresses significant to individual hosts
  • Loopback Address
• Special Addresses of Global Significance
  • Private Addresses
  • Reserved Addresses

IP Subnets

NETWORK ADDRESS

A network address is an address where all host bits in the IP address are set to zero (0).

In every subnet there is a network address. This is the first and lowest numbered address in the range because the address is always the address where all host bits are set to zero. The network address is defined in the RFC's as as the address that contains all zeroes in the host portion of the address and is used to communicate with devices that maintain the network equipment.

Today it is rare to see the network address in use.

BROADCAST ADDRESS

A broadcast address is an address where all host bits in the IP address are set to one (1).

This address is the last address in the range of addresses, and is the address whose host portion is set to all ones. All hosts are to accept and respond to the broadcast address. This makes special services possible.

Hosts

LOOPBACK ADDRESS (127.0.0.1)

The 127.0.0.0 class 'A' subnet is used for special local addresses, most commonly the loopback address 127.0.0.1. This address is used to test the local network interface device's functionality. All network interface devices should respond to this address from the command line of the local host. If you ping 127.0.0.1 from the local host, you can be assured that the network hardware is functioning and that the network software is also functioning. The addresses in the 127.0.0.0 - 127.255.255.255 range cannot be reached from outside the host, and so cannot be used to build a LAN.

Special Use IP addresses

PRIVATE IP ADDRESSES

RFC 1918 defines a number of IP blocks which were set aside by the American Registry of Internet Numbers (ARIN) for use as private addresses on private networks that are not directly connected to the Internet. The private addresses are:

Multicast IP Addresses

There are a number of addresses that are set aside for special purposes, such as the IP's used in OSPF, Multicast, and experimental purposes that cannot be used on the Internet.

Special Use Addresses - Table from RFC 3330

Address Block	CIDR Mask	Used for	Reference
0.0.0.0	/8	Used to communicate with "This" network	RFC1700, p. 4
10.0.0.0	/8	Private-Use Networks	RFC 1918
14.0.0.0	/8	Public-Data Network	RFC1700, p.181
24.0.0.0	/8	Cable TV Networks	--
39.0.0.0	/8	Previously Reserved Available for Regional Allocation	RFC1797
127.0.0.0	/8	Loopback address	RFC1700, p. 5
128.0.0.0	/16	Previously Reserved Available for Regional Allocation	--
169.254.0.0	/16	Link Local (eg. Microsoft XP systems use Automatic Private IP Addressing (APIPA) which selects addresses in this range.)
172.16.0.0	/12
191.255.0.0	/16
192.0.0.0	/16
192.88.99.0	/16
192.168.0.0	/16