Internet Addresses and Subnets

The following is an example of a subnet IP address you might have on your computer at home if you're using a router (wireless or wired) between your ISP connection and your computer:

  • IP address: 192.168.1.102
  • Subnet mask: 255.255.255.0
  • Twenty-four bits (three octets) reserved for network identity
  • Eight bits (one octet) reserved for nodes
  • Subnet identity based on subnet mask (first address): 192.168.1.0
  • The reserved broadcast address for the subnet (last address): 192.168.1.255
  • Example addresses on the same network: 192.168.1.1, 192.168.1.103
  • Example addresses not on the same network: 192.168.2.1, 192.168.2.103

Besides reserving IP addresses, the IANA is also responsible for assigning blocks of IP addresses to certain entities, usually commercial or government organizations. Your Internet service provider (ISP) may be one of these entities, or it may be part of a larger block under the control of one of those entities. In order for you to connect to the Internet, your ISP will assign you one of these addresses. You can see a full list of IANA assignments and reservations for IPv4 addresses here.

If you only connect one computer to the Internet, that computer can use the address from your ISP. Many homes today, though, use routers to share a single Internet connection between multiple computers. Wireless routers have become especially popular in recent years, avoiding the need to run network cables between rooms.

If you use a router to share an Internet connection, the router gets the IP address issued directly from the ISP. Then, it creates and manages a subnet for all the computers connected to that router. If your computer's address falls into one of the reserved subnet ranges listed earlier, you're going through a router rather than connecting directly to the Internet.

IP addresses on a subnet have two parts: network and node. The network part identifies the subnet itself. The node, also called the host, is an individual piece of computer equipment connected to the network and requiring a unique address. Each computer knows how to separate the two parts of the IP address by using a subnet mask. A subnet mask looks somewhat like an IP address, but it's actually just a filter used to determine which part of an IP address designates the network and node.

A subnet mask consists of a series of 1 bits followed by a series of 0 bits. The 1 bits indicate those that should mask the network bits in the IP address, revealing only those that identify a unique node on that network. In the IPv4 standard, the most commonly used subnet masks have complete octets of 1s and 0s as follows:

  • 255.0.0.0.0 = 11111111.00000000.00000000.00000000 = eight bits for networks, 24 bits for nodes
  • 255.255.0.0 = 11111111.11111111.00000000.00000000 = 16 bits for networks, 16 bits for nodes
  • 255.255.255.0 = 11111111. 11111111.11111111.00000000 = 24 bits for networks, eight bits for nodes

People who set up large networks determine what subnet mask works best based on the number of desired subnets or nodes. For more subnets, use more bits for the network; for more nodes per subnet, use more bits for the nodes. This may mean using non-standard mask values. For instance, if you want to use 10 bits for networks and 22 for nodes, your subnet mask value would require using 11000000 in the second octet, resulting in a subnet mask value of 255.192.0.0.

Another important thing to note about IP addresses in a subnet is that the first and last addresses are reserved. The first address identifies the subnet itself, and the last address identifies the broadcast address for systems on that subnet.

See the sidebar on the previous page for a look at how all this information comes together to form your IP address. For even more information about IP addresses, the Internet and related networking topics, head on to the next page.