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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 characters, and the network mask to use on that network.
     Network  numbers  and  masks may be specified in the conven-
     tional 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 identi-
     cal.  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), 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  net-
     masks file. For example,


SunOS 5.9            Last change: 7 Jan 1997                    1

File Formats                                          netmasks(4)


     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 Interna-
     tional, Menlo Park, Calif., August 1985.

     V. Fuller, T. Li, J. Yu, K. Varadhan, Classless Inter-Domain
     Routing  (CIDR): an Address Assignment and Aggregation Stra-
     tegy, 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  Information Center, SRI Interna-
     tional, 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 compa-

SunOS 5.9            Last change: 7 Jan 1997                    2