NETMASKS(5) File Formats and Configurations NETMASKS(5)

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.

January 7, 1997 NETMASKS(5)