NETMASKS(5) Standards, Environments, and Macros NETMASKS(5)


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 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. 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(8), 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

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.


inet(4P), ifconfig(8)

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 International, Menlo Park, Calif., September

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.

January 7, 1997 NETMASKS(5)