Where & why 31 and 32 subnet mask are used?

Hi, recently I brought IDEA NETSEETER for accessing Internet. When I check details of it I saw that the subnet mask is or /32. I search on net and found that /31 and /32 are allowed to use for some special cases. But did not get a clear picture.

In normal condition we can not use /31 and /32 subnet mask. So my question are:

1) In what condition /31 and /32 are use?
2) And what are network and broadcast address in these case.


  • durga ch
    durga ch
    one example I can think of currently
    ISP <-->R1 --(your network)
    as per addressing rules, the segment between ISP and R1 need to be in the SAME network. Many times this will be a serial link and due to which you need only 2 addresses for each side. which will be a /30 network address. (2^2 -2 =2 usable addresses)
    but again there are few theories/RFCs. For point to point ( manily used in frame relay or Ppp) we 'dont' really end up broadcasting. 😀 as we have only 2 endpoints, hence it doesnot make a point to have a reserved 'broadcast' address. think of it like a water pipe moving water from tap to a drum. you wont see water spilling everywhere, it only flows in a gien channel. same is the case with point to point. You dont need a broadcast address. hence /31 address can also be used giving scope to 2 host address ,(2^1) - ie. host bit '0' represents one host and '1' represents another.
    /32 is used to represent a host as it is.
  • Kishan Gupta
    Kishan Gupta
    Thanks Durga for your reply.

    Still I have one question , in case of 3g modems( in all dial up) /32 is used, then how it communicate to server?
  • durga ch
    durga ch
    I an not too familiar with dailup configuration, but have another example for you for /32 addresses
    you must have heard of classical loopback interfaces . right? Those can be treated as single /32 address.
    Loopback interfaces generally are used to test the tcp stack within a given host , like -

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