Netmasks and Subnetting
Everyone seems to be confused by netmasks and all of the different formats used. I remember when I was first trying to learn how netmasks and subnetting work. Although I was able to understand it right away, it was very tricky...until a friend of mine explained it to me in a different way. After he explained it, it was simple!

We are starting out assuming that you already have a bit of networking knowledge. This page isn't meant for people who have no idea what a netmask is, but if you read through it all you should be able to figure it out.

The 'slash' notation Ever see an IP address with a modifier on the end such as 211.14.238.4/29. Well, if your like me you probably can't see how they came up with the /29. Ok, remember the current numbering system on the internet is a 32 bit system. The /29 is just the number of bits that get 'sent through' the route. Please note that Cisco's "auto install" does the bits a bit backwards. I have noted the CISCO number after the dash's for you CISCO people.

Common Subnet Notations

/Bits - CISCO# # of IP addresses Netmask (256 - bit's)
/32 - 8 1 255.255.255.255
/31 - 7 2 255.255.255.254
/30 - 6 4 255.255.255.252
/29 - 5 8 255.255.255.248
/28 - 4 16 255.255.255.240
/27 - 3 32 255.255.255.224
/26 - 2 64 255.255.255.192
/25 - 1 128 255.255.255.128
/24 - 0 256 255.255.255.0

How did we figure this out? It was actually quite easy, look...

Number of addresses - Addresses being routed = netmask!
256 - 1 = 255
256 - 2 = 254
256 - 4 = 252
256 - 8 = 248
256 - 16 = 240
256 - 32 = 224
256 - 64 = 192
256 - 128 = 128
256 - 256 = 0