RFC for subnet mask

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andrew.burns Thu, 01/03/2008 - 00:23
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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.


andrew.burns Thu, 01/03/2008 - 00:57
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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.


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