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 220.127.116.11
should have eight bits of subnet field and eight bits of
host field, in addition to the standard sixteen bits in the
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,
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File Formats netmasks(4)
can be used to specify different netmasks in different parts
of the 18.104.22.168 Class B network number. Addresses
22.214.171.124 through 126.96.36.199 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 188.8.131.52 through
184.108.40.206 have a 26 bits in the subnet field. Finally,
all other addresses in the range 220.127.116.11 through
18.104.22.168 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 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-
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