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netmasks(4)                      File Formats                      netmasks(4)

       netmasks - network mask database



       The  netmasks  file contains network masks used to implement IP subnet-
       ting. It supports both standard subnetting as specified in RFC-950  and
       variable  length  subnetting as specified in RFC-1519. When using stan-
       dard subnetting there should be a single line for each network that  is
       subnetted  in this file with the network number, any number of SPACE or
       TAB characters, and the network mask to use on  that  network.  Network
       numbers  and  masks  may  be specified in the conventional IP `.' (dot)
       notation (like IP host addresses, but with zeroes for the  host  part).
       For example,


       can  be used to specify that the Class B network should have
       eight bits of subnet field and eight bits of host field, in addition to
       the standard sixteen bits in the network field.

       When  using  variable  length subnetting, the format is identical. How-
       ever, there should be a line for each  subnet  with  the   first  field
       being the subnet and the second field being the netmask that applies to
       that subnet. The users of the database, such as ifconfig(1M), perform a
       lookup  to  find  the longest possible matching mask. It is possible to
       combine the  RFC-950 and  RFC-1519 form of subnet masks in the netmasks
       file. For example,


       can  be  used  to  specify different netmasks in different parts of the  Class  B  network  number.  Addresses  through  have  a subnet mask with 28 bits in the combined network
       and subnet fields  (often referred to as the subnet field) and  4  bits
       in   the   host  field.   Furthermore,  addresses  through have a 26 bits in the subnet  field.  Finally,  all  other
       addresses  in the range through have a 24 bit
       subnet field.

       Invalid entries are ignored.

       ifconfig(1M), inet(7P)

       Postel, Jon, and Mogul, Jeff, Internet Standard  Subnetting  Procedure,
       RFC  950,  Network  Information  Center, SRI International, Menlo Park,
       Calif., August 1985.

       V. Fuller, T. Li, J. Yu, K. Varadhan, Classless  Inter-Domain   Routing
       (CIDR):  an Address Assignment and Aggregation Strategy, RFC 1519, Net-
       work Information Center, SRI International, Menlo Park, Calif., Septem-
       ber 1993.

       T.  Pummill,  B.  Manning,  Variable  Length Subnet Table For IPv4, RFC
       1878,  Network  Information  Center,  SRI  International,  Menlo  Park,
       Calif., December 1995.

       /etc/inet/netmasks is the official SVr4 name of the netmasks file.  The
       symbolic link /etc/netmasks exists for BSD compatibility.

SunOS 5.10                        7 Jan 1997                       netmasks(4)