How does a subnet work?

First, a bit about IP subnetting. With your IP address, you have a subnet mask. This subnet mask determines which IP addresses are located on your local network. You also have a default gateway, which is the network device (typically a router or firewall) that the machine knows to contact for any machine that is not on the local network.

The most common subnet mask on private networks is 255.255.255.0, which means the last octet of the IP address, like 192.168.1.X above, can be 1 through 254. This means once you get above 254 devices on your network, you run out of address space on a network using a 255.255.255.0 subnet mask.

At this point, your options are to either change your subnet mask, or put in a router. Which is most appropriate depends on plans for network growth. In a fast growing network, or one with a significant amount of network traffic, a layer 3 switch (a switch that acts as a very fast router) is the most appropriate choice. In a network that won't get many more than 275-300 hosts and doesn't have a whole lot of network traffic, using a larger subnet mask is an acceptable choice.

To increase the number of IP's on the subnet, change the subnet mask on *all* hosts to 255.255.252.0, while still using the same IP addresses. Make sure every single host gets changed, or you will have communication issues. With a 255.255.252.0 mask, your network will now range from 192.168.0.0 through 192.168.3.254.

There is substantially more to IP subnetting and network design than could possibly be described here. Inappropriate subnetting and design can lead to severe network performance problems, so I strongly suggest if you don't have expertise in this area that you contact a qualified network consultant experienced in enterprise network design.

Suggested resources:

• Network Computing's IP 101
• IP Subnets
• Cisco's Subnet Tutorial