User's Manual
Appendix A IP Addresses and Subnetting
B222s User’s Guide
181
Subnet m asks can be referred to by t he size of t he network num ber part ( the bit s wit h a “ 1” value) .
For exam ple, an “ 8-bit m ask” m eans t hat t he first 8 bit s of t he m ask are ones and t he rem aining 24
bits are zeroes.
Subnet m asks are expressed in dot ted decim al not at ion j ust like I P addresses. The following
exam ples show t he binary and decim al notat ion for 8- bit , 16- bit , 24- bit and 29- bit subnet m asks.
Network Size
The size of the network num ber det erm ines t he m axim um num ber of possible hosts you can have
on your network. The larger t he num ber of net work num ber bits, the sm aller the num ber of
rem aining host I D bits.
An I P address wit h host I Ds of all zeros is the I P address of t he net work ( 192.168.1.0 wit h a 24- bit
subnet m ask, for exam ple) . An I P address with host I Ds of all ones is t he broadcast address for t hat
network ( 192.168.1.255 wit h a 24- bit subnet m ask, for exam ple).
As t hese t wo I P addresses cannot be used for individual hosts, calculate the m axim um num ber of
possible host s in a network as follows:
Notation
Since the m ask is always a cont inuous num ber of ones beginning from the left, followed by a
continuous num ber of zeros for t he rem ainder of t he 32 bit m ask, you can sim ply specify t he
num ber of ones instead of writing the value of each octet. This is usually specified by w rit ing a “ / ”
follow ed by the num ber of bits in t he m ask aft er the address.
For exam ple, 192.1.1.0 / 25 is equivalent t o saying 192.1.1.0 wit h subnet m ask 255.255.255.128.
Table 75 Subnet Masks
BINARY
DECIMAL
1ST OCTET 2ND OCTET 3RD OCTET 4TH OCTET
8-bit m ask 11111111 00000000 00000000 00000000 255.0.0.0
16- bit m ask 11111111 11111111 00000000 00000000 255.255.0.0
24- bit m ask 11111111 11111111 11111111 00000000 255.255.255.0
29- bit m ask 11111111 11111111 11111111 11111000 255.255.255.248
Table 76 Maxim um Host Num bers
SUBNET MASK HOST ID SIZE
MAXIMUM NUMBER OF
HOSTS
8 bits 255.0.0.0 24 bit s 2
24
– 2 16777214
16 bit s 255.255.0.0 16 bit s 2
16
– 2 65534
24 bit s 255.255.255.0 8 bits 2
8
– 2 254
29 bit s 255.255.255.248 3 bit s 2
3
– 2 6