ZyXEL Communications VMG1312-B Series, User Manual

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Appendix B IP Addresses and Subnetting
VMG1312-B Series User’s Guide
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Subnet m asks can be referred t o by t he size of t he net w ork 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.
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:
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 “ / ”
follow ed 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.
Table 112 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 113 Maxim um Host Num bers
SUBNET MASK HOST ID SIZE MAXIMUM NUMBER OF HOSTS
8 bit s 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 bit s 2
8
– 2 254
29 bit s 255.255.255.24
8 3 bit s 2
3
– 2 6