RFC for subnet mask

[aksher]

Which RFC describes the illegal subnet masks (255.255.255.254, 255.255.255.255)?

[andrew.burns]

Hi,

Those are not "illegal" subnet masks. The first can be used in access-lists to match exactly two IP addresses, and the second matches exactly one IP address and is the usual subnet mask for a loopback interface.

HTH

Andrew.

[aksher]

Am familiar with these things but when it comes to design a network i get this doubt quite often?

[andrew.burns]

Hi,

RFC 4632 would probably make useful reading. To quote section 3.1 :

"In CIDR notation, a prefix is shown as a 4-octet quantity, just like a traditional IPv4 address or network number, followed by the "/" (slash) character, followed by a decimal value between 0 and 32 that describes the number of significant bits."

So, this basically says your subnet mask can be any value from 0 (i.e. a default route) to 32 (i.e. a "host"). In your example 255.255.255.254 is 31 bits and 255.255.255.255 is 32 bits so both are valid.

There's a nice table that illustrates this in the RFC as well.

HTH

Andrew.