ZyXEL Communications VMG1312-B10A - Network Size

Appendix B IP Addresses and Subnetting
VMG1312-B Series User’s Guide
321
Subnet m asks can be referred t o by t he size of the net work num ber part (t he bits with a “ 1” value) .
For exam ple, an “ 8- bit m ask” m eans that the first 8 bits of t he m ask are ones and t he rem aining 24
bit s are zeroes.
Subnet m asks are expressed in dott ed decim al not ation just like I P addresses. The following
exam ples show the binary and decim al not ation for 8- bit, 16-bit, 24- bit and 29-bit subnet masks.
Network Size
The size of t he network num ber determ ines the m axim um num ber of possible host s you can have
on your network. The larger t he num ber of network num ber bit s, t he sm aller the num ber of
rem aining host I D bits.
An I P address with host I Ds of all zeros is t he I P address of t he network ( 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
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 hosts, calculate t he m axim um num ber of
possible host s in a net w ork as follows:
Notation
Since the m ask is always a cont inuous num ber of ones beginning from the 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 the
num ber of ones inst ead of writing the value of each oct et. This is usually specified by writ ing a “ / ”
followed by t he num ber of bits 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 mask 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 bits 255.0.0.0 24 bits 2
24
– 2 16777214
16 bit s 255.255.0.0 16 bits 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 bits 2
3
– 2 6