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Subnet Representations and Aliases

Last Updated: Oct 07, 2014 08:38PM PDT
Anybody who has manually configured their network settings has had to enter a subnet mask, usually in the form of “#.#.#.#”. The most familiar subnet value for many is 255.255.255.0, representing that the first three segments of the associated IP address must be the same as another IP address for the other address to match the subnet. The behaviour of this subnet is almost self-evident due to the syntax. However, this syntax can be misleading. What does a subnet of 255.240.0.0 mean? Or the subnet 255.192.0.0? And why is the subnet 255.127.0.0 invalid?

An alternative, if not more sensible, representation of subnets is the slash notation, or “/n” where ‘n’ is the ‘mask length’ – the number of bits that must match between two IP addresses to make them part of the same subnet.

Consider the following:

     IP 192.168.0.1
          (binary: 11000000 10101000 00000000 00000001)
     Subnet 255.255.255.0
          (binary: 11111111 11111111 11111111 00000000)

This would be the same as 192.168.0.1/24, as 255.255.255.0 has the first 24 bits turned on, meaning anything that starts with 192.168.0.x will match it.

A /16 is equivalent to 255.255.0.0, as only the first 16 bits are on. 192.168.0.1/16 would match anything between 192.168.0.0 and 192.168.255.255.

It gets a bit messy with mask lengths that aren’t multiples of 8, for example 10.0.0.1/12 (/12 is equivalent to 255.240.0.0) would match anything between 10.0.0.0 and 10.15.255.255, and 10.16.0.1/12 would match anything between 10.16.0.0 and 10.31.255.255. This is because the addresses must match all 8 bits of the first segment (10), and only the most significant 4 bits of the second segment (16), making for a total match of 12 bits, or a /12 subnet. For example;

     IP 10.0.0.1
          (00001010 00000000 00000000 00000001)
     IP 10.16.0.1
          (00001010 00010000 00000000 00000001)
     Subnet 255.240.0.0 (/12)
          (11111111 11110000 00000000 00000000)

In effect, the shown bits must be the same for the /12 subnet to match an IP in that subnet range;

     IP/Subnet 10.16.0.0/12
          (00001010 0001xxxx xxxxxxxx xxxxxxxx)
     IP 10.16.1.5 matches 10.16.0.0/12
          (00001010 00010000 00000001 00000101)
     IP 10.19.3.1 matches 10.16.0.0/12
          (00001010 00010011 00000011 00000001)
     IP 10.31.1.1 matches 10.16.0.0/12
          (00001010 00011111 00000001 00000001)
     IP 10.32.1.1 does not match 10.16.0.0/12
          (00001010 00100000 00000001 00000001)
     IP 10.0.1.1 does not match 10.16.0.0/12
          (00001010 00000000 00000001 00000001)

WebSpy Vantage uses the slash notation to represent subnet values in Subnet Aliases because it is easier to understand and represent. The slash notation is also used to represent IPv6 subnets, where there is no suitable analogue to the #.#.#.# representation of IPv4 subnets.

I found a subnet calculator that converts between the slash notation and the #.#.#.# representation at http://www.nettoolsonline.com/slash_conversion.php, which comes in handy if you don’t want to have to do the maths yourself.

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