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OpenSolaris 2009.06 - man page for netmasks (opensolaris section 4)

netmasks(4)				   File Formats 			      netmasks(4)

       netmasks - network mask database



       The netmasks file contains network masks used to implement IP subnetting. It supports both
       standard subnetting as specified in RFC-950 and variable length subnetting as specified in
       RFC-1519.  When	using  standard 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 charac-
       ters, and the network mask to use on that network. Network numbers and masks may be speci-
       fied 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 sub-
       net 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. However, 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), per-
       form 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, Network Information  Center,  SRI  Interna-
       tional, Menlo Park, Calif., September 1993.

       T.  Pummill, B. Manning, Variable Length Subnet Table For IPv4, RFC 1878, Network Informa-
       tion 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 5.11				    7 Jan 1997				      netmasks(4)

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