ZyXEL Communications VMG1312-B Series User Manual page 321

Subnet m asks can be referred t o by t he size of t he net work num ber part ( t he 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
bit s are zeroes.
Subnet m asks are expressed in dot t ed decim al not at ion j ust like I P addresses. The following
exam ples show t he binary and decim al not at ion for 8- bit , 16- bit , 24- bit and 29- bit subnet m asks.
Table 112 Subnet Masks
8- bit m ask
16- bit m ask
24- bit m ask
29- bit m ask
Network Size
The size of t he net work num ber det erm ines t he m axim um num ber of possible host s you can have
on your net work. The larger t he num ber of net work num ber bit s, t he sm aller t he num ber of
rem aining host I D bit s.
An I P address wit h host I Ds of all zeros is t he 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 wit h host I Ds of all ones is t he broadcast address for t hat
net work ( 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 host s, calculat e t he m axim um num ber of
possible host s in a net work as follows:
Table 113 Maxim um Host Num bers
SUBNET MASK
8 bit s 255.0.0.0
16 bit s 255.255.0.0
24 bit s 255.255.255.0
29 bit s 255.255.255.24
8
Notation
Since t he m ask is always a cont inuous num ber of ones beginning from t he left , followed by a
cont inuous 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 inst ead of writ ing t he value of each oct et . This is usually specified by writ ing a " / "
followed by t he num ber of bit s in t he m ask aft er t he 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.
VMG1312-B Series User's Guide
BINARY
1ST 2ND
OCTET OCTET
11111111 00000000
11111111 11111111
11111111 11111111
11111111 11111111
HOST ID SIZE
24 bit s
16 bit s
8 bit s
3 bit s
Appendix B IP Addresses and Subnetting
3RD
4TH OCTET
OCTET
00000000 00000000
00000000 00000000
11111111 00000000
11111111 11111000
MAXIMUM NUMBER OF HOSTS
24
2 – 2 16777214
16
2 – 2 65534
8
2 – 2 254
3
2 – 2 6
DECIMAL
255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.248
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