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 188.8.131.52 should have eight bits of sub-
net field and eight bits of host field, in addition to the standard sixteen bits in the
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 184.108.40.206 Class B
network number. Addresses 220.127.116.11 through 18.104.22.168 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 22.214.171.124 through 126.96.36.199
have a 26 bits in the subnet field. Finally, all other addresses in the range 188.8.131.52
through 184.108.40.206 have a 24 bit subnet field.
Invalid entries are ignored.
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)