Defining an Internet Address

Absolutely every device that exists on the internet needs an IP address (some call it an internet address). There are no exceptions – every pc, laptop, phone, coffee machine or printer if it’s online and accessible it needs an IP address. Needless to say the internet facing ones have to be unique in order to communicate without problems and hence the numbers need to be quite long. An IP address is actually a 32 bit number, so instead of just being a flat linear number, an IP address range has structure.  The addresses fit into five categories as follows –

  1. Class A – Netid 7 bits, hostid 24 bits
  2. Class B – netid 14 bits, hostid 16 bits
  3. Class C – netid 21 bits, hostid 8 bits
  4. Class D – multicast group – 28 bits id
  5. Class E – reserved for future use

The 32 bit addresses in most cases are written in decimal notation as four numbers, each number specifies a byte of the address.

You’ll commonly see this address format referred to as the dot-decimal notation.  You’ll have seen these addresses everywhere – something like Although we have noted that every device on the internet will have an IP address assigned to it, some may have more than one.

For example a server may have multiple network interface cards and each one needs a seperate address. These are called multihome devices and are very common in large networks. Obviously as each address must be unique, there must be someone in charge of assigning these addresses in order to avoid duplication. The authority that is in charge of these is called the InterNIC – the Internet Network Information Centre.  They are in charge of ordering network ids, the allocation of individual addresses is left to the system administrators.

These addresses are allocated logically by area, so an administrator in Britain will get assigned a block and each device will be allocated a British IP address (source:, this allows web sites to track visitors using geotargeting technology. Commonly used to restrict or tailor access to web clients – e.g. only access Hulu in the US or watch RTE player outside Ireland. There is more to understanding IP addresses, but hopefully this is a start. In future posts we’ll cover the different types of addresses like broadcast, multicast and unicast.  Also a look at how we can route IP addresses and concepts like subnetting.