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Appendix C IP Netmask
IP Netmask or Subnet Mask is a 32-bit pattern of ones and zeros used to determine
network portion of an IP address from the host portion of the IP address. Subnet mask is
a network ID that is created by borrowing bits from host portion of IP address and using
them as part of a network ID. The table below shows a default subnet mask for address
Classes A, B, and C. Each bit that is set to "1" in the subnet mask corresponds to the bit
in the IP address that is to be used as the network ID. Each bit that is set to "0" in the
subnet mask corresponds to a bit in the IP address that is to be used as the host ID.
Address Class Mask Binary Value Mask Decimal Value
or Dotted Notation
Class A 255.0.0.0
Class B 255.255.0.0
Class C 255.255.255.0
If your network requires more network ID’s, you can extend the default subnet mask to
include additional bits from the host ID. This allows for additional network ID’s within the
network. The table below shows some examples of subnet masks and bits moved from
the hosts ID to create a new subnet.
Mask Dotted Notation Mask Binary Mask Bits
Class A
255.0.0.0 (Default) 0
255.192.0.0 2
255.224.0.0 3
255.240.0.0 4
255.248.0.0 5
255.252.0.0 6
255.254.0.0 7
255.255.0.0 8
255.255.128.0 9
255.255.192.0.0 10
……………......... .
255.255.255.252 22
Class B
255.255.0.0 (Default) 0
255.255.192.0 2
……………......... .
255.255.255.252 14
Class C
255.255.255.0 (Default) 0
255.255.255.192 2
…………………. .
255.255.255.254 6
To determine the number of valid hosts ID’s remaining after subnetting, use the following
equation: 2
n
– 2, where n is the number of octet digits left after the subnet mask.
40
11111111
11111111
11111111
00000000
11111111
11111111
00000000
00000000
11111111
00000000
00000000
00000000
11111111
11111111
11111111
11111111
11111111
11111111
11111111
11111111
11111111
11111111
........
11111111
00000000
11000000
11100000
11110000
11111000
11111100
11111110
11111111
11111111
11111111
........
11111111
00000000
00000000
00000000
00000000
00000000
00000000
00000000
00000000
10000000
11000000
........
11111111
00000000
00000000
00000000
00000000
00000000
00000000
00000000
00000000
00000000
00000000
........
11111100
11111111
11111111
........
11111111
11111111
11111111
........
11111111
00000000
11000000
........
11111111
00000000
00000000
........
11111100
11111111
11111111
........
11111111
11111111
11111111
........
11111111
11111111
11111111
........
11111111
00000000
11000000
........
11111100