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Appendix E IP Addresses and Subnetting
User’s Guide
339
By convention, subnet masks always consist of a continuous sequence of ones
beginning from the leftmost bit of the mask, followed by a continuous sequence of
zeros, for a total number of 32 bits.
Subnet masks can be referred to by the size of the network number part (the bits
with a “1” value). For example, an “8-bit mask” means that the first 8 bits of the
mask are ones and the remaining 24 bits are zeroes.
Subnet masks are expressed in dotted decimal notation just like IP addresses. The
following examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit
and 29-bit subnet masks.
Network Size
The size of the network number determines the maximum number of possible
hosts you can have on your network. The larger the number of network number
bits, the smaller the number of remaining host ID bits.
An IP address with host IDs of all zeros is the IP address of the network
(192.168.1.0 with a 24-bit subnet mask, for example). An IP address with host
IDs of all ones is the broadcast address for that network (192.168.1.255 with a
24-bit subnet mask, for example).
As these two IP addresses cannot be used for individual hosts, calculate the
maximum number of possible hosts in a network as follows:
Table 128 Subnet Masks
BINARY
DECIMAL
1ST
OCTET 2ND
OCTET 3RD
OCTET 4TH
OCTET
8-bit mask 11111111 00000000 00000000 00000000 255.0.0.0
16-bit
mask 11111111 11111111 00000000 00000000 255.255.0.0
24-bit
mask 11111111 11111111 11111111 00000000 255.255.255.0
29-bit
mask 11111111 11111111 11111111 11111000 255.255.255.24
8
Table 129 Maximum Host Numbers
SUBNET MASK HOST ID SIZE
MAXIMUM NUMBER OF
HOSTS
8 bits 255.0.0.0 24 bits 2
24
– 2 16777214
16 bits 255.255.0.0 16 bits 2
16
– 2 65534
24 bits 255.255.255.0 8 bits 2
8
– 2 254
29 bits 255.255.255.2
48
3 bits 2
3
– 2 6