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 18.104.22.168/29. 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|
|Number of addresses||-||Addresses being routed||=||netmask!|