Your computer doesn't care if it's on a Class C subnet or a Class A. Your computer doesn't care if you are using 10.1.1.0, 172.16.38.0 or 192.168.1.0 as your subnet. If you've set up everything else to match, it's all going to work.
However, human beings are easily confused, especially when it comes to multiple subnets. For example, let's say you have a remote office and you need to set up a VPN between that and your main office. Your main office currently uses 192.168.1.0 (a class C network). What should the branch office use?
If you have no idea what I'm talking about, see Networking 101 here.
You could use 192.168.2.0. As noted, your computers won't care - as long as it's a different subnet, the specific choice doesn't matter. However, I'd suggest doing something more helpful for humans.
For example, I'd either use 192.168.100.0 or 10.1.100.0 or 172.16.100.0 for the remote office because the addresses will be more obviously different to humans like me. The 10.1.100.0 is markedly different, but the 192.168.100.0 and 172.16.100.0 also stand out because the "100" is two digits longer than the "1".
If it makes sense to do so, I'd also select a different range for servers and DHCP cients. If the main office uses 192.168.1.1 through 192.168.1.100 for machines, I might use 192.168.100.101 through 192.168.100.200 at the remote - again, this is only to make it more obvious to humans looking at network reports and tools.
I would also avoid mixing Class B and Class C networks if at all possible. Unless you must use Class B, I'd stay clear of that entirely because sooner or later something will get configured as 255.255.255.0 and cause confusion.
Remember - none of this matters at a technical level. It can just help avoid human mistakes and confusion.
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More Articles by Anthony Lawrence © 2012-07-14 Anthony Lawrence