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 |
| 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 |