Special IP Addresses

Special Addresses

The following are special IP addresses reserved for use during the operation of an IP based network.

Networks / Subnets

• A network/subnet number with all binary 0s - This specifies "this" network/subnet ie. the current network/subnet.

• A subnet number with all binary 1s - This specifies all subnets.

As you cannot have a subnet number of 0 (all binary zeros) or 255 (all binary 1s), you can only have 254 subnets when using 1 octet for subnetting (ie. 2^8 -2 = 254).

Hosts

• A host portion with all binary 1s - Sending a packet to this address will broadcast the packet to all hosts on the specified network/subnet.

• A host portion with all binary 0s - This address specifies a network/subnetwork, but does not specify any host on the network/subnetwork. This form is commonly used in routing tables.

As you cannot have a host number of 0 (all binary zeros) or 255 (all binary 1s), you can only have 254 hosts when using 1 octet for host addresses (ie. 2^8 - 2 = 254).

Individual Networks

• 127.0.0.0 - This special Class A network is commonly known as the "loopback" network. Any packets sent to a host on this network will not be sent out the network card installed in the computer. They will be returned to back to the machine. This allows both a client application and server application to reside on the same machine. The 127.0.0.0 is not, nor should not be subnetted. Most operating systems will automatically configure this address.

Individual IP Addresses

• Network/subnet/host portions with all binary 1s - 255.255.255.255 - This address can only be used as a source IP address, to send a broadcast to all hosts on the current network.

• A network/subnet portion with all binary 0s, and a specified host portion eg. 0.0.0.5 - this address specifies a host on "this" network ie. the network the host is currently attached to.

• Network/subnet/host portions with all binary 0s - 0.0.0.0 - This address can only be used as a source IP address, when the host does not know its IP address ie. it has not been assigned. It is also used in route tables to indicate a default route.

RFC 1918 Addresses

The people at InterNIC realise that there may be organisations who want to build their own private IP based internetworks, using any old IP addressing scheme.

However, if these organisations connected to the Internet at a later date, the addresses they may have used could conflict with official Internet addresses.

To remedy this, InterNIC have allocated some Class A, B and C network addresses for use in private networks, which would not cause problems if accidently connected to the Internet. Keep in mind, these addresses will not work on the Internet either.

These network addresses are documented in RFC 1918. They are :

• Class A network 10.0.0.0 - 1 Class A

• Class B networks 172.16.0.0 to 172.31.0.0 - 16 Class Bs

• Class C networks 192.168.0.0 to 192.168.255.0 - 255 Class Cs

If you are going to create your own TCP/IP network segment using ethernet at home, and still have a machine dialing into the Internet, it is best to use one of the above Class A, B or C networks.