Q:

How can I find out if a host IP address is valid or invalid?

A:

A basic understanding of what IP addresses represent can help to determine their validity. There are also free utilities available on the web that allow users to enter a specific IP address to determine the network and broadcast addresses of a given IP.

An IP address is a 32-bit code that identifies the computer or other device to a TCP/IP network. The 32-bit code consists of four octets, also known as IPv4. The octets are converted to a dotted decimal format, resulting in a series of four numbers separated by dots. The first three numbers represent the network. The last number represents the host.

Valid IP addresses follow certain protocols.

  • Class A or large networks: 001.0.0.001 to 126.254.254.254
  • Class B or medium sized networks: 128.1.0.1 to 191.254.254.254
  • Class C or small networks: 192.0.1.1 to 223.254.254.254

There are certain clues that immediately indicate that an IP address is invalid.

  • The number 255 should not appear in any of the octets.
  • The number 0 cannot be the first or the last number (host number) in the IP address.
  • Exception: the number 0.0.0.0 is used by hosts which do not know their IP address.

The number 255 is reserved for broadcast addressing and is used in the subnet mask, which is a code used by a TCP/IP network to determine if the host is on a remote network or not.


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