man/man5/netmasks.5

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 Copyright 1989 AT&T
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 NETMASKS 5 "Jan 7, 1997"
 NAME
netmasks - network mask database
 SYNOPSIS

/etc/inet/netmasks


/etc/netmasks


 DESCRIPTION

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,


128.32.0.0 255.255.255.0


can be used to specify that the Class B network 128.32.0.0 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 example,


128.32.0.0 255.255.255.0
128.32.27.0 255.255.255.240
128.32.27.16 255.255.255.240
128.32.27.32 255.255.255.240
128.32.27.48 255.255.255.240
128.32.27.64 255.255.255.240
128.32.27.80 255.255.255.240
128.32.27.96 255.255.255.240
128.32.27.112 255.255.255.240
128.32.27.128 255.255.255.240
128.32.27.144 255.255.255.240
128.32.27.160 255.255.255.240
128.32.27.176 255.255.255.240
128.32.27.192 255.255.255.240
128.32.27.208 255.255.255.240
128.32.27.224 255.255.255.240
128.32.27.240 255.255.255.240
128.32.64.0 255.255.255.192


can be used to specify different netmasks in different parts of the 128.32.0.0
Class B network number. Addresses 128.32.27.0 through 128.32.27.255 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 128.32.64.0 through 128.32.64.63 have a 26 bits in the subnet field.
Finally, all other addresses in the range 128.32.0.0 through 128.32.255.255
have a 24 bit subnet field.

Invalid entries are ignored.
 SEE ALSO

 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 1993.

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

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