ZyXEL Communications VMG1312-B Series - Subnet Masks

Appendix B IP Addresses and Subnetting
VMG1312-B Series User’s Guide
320
The following figure shows an exam ple I P address in which the first three oct et s (192.168.1) are
the network num ber, and the fourt h octet ( 16) is t he host I D.
Figure 181 Networ k Num ber and Host I D
How m uch of the I P address is t he network num ber and how m uch is the host I D varies according
to the subnet m ask.
Subnet Masks
A subnet m ask is used t o det erm ine which bits are part of the network num ber, and which bits are
part of the host I D (using a logical AND operation). The t erm “subnet ” is short for “ sub- network”.
A subnet m ask has 32 bit s. I f a bit in t he subnet m ask is a “ 1” then the corresponding bit in the I P
address is part of the network num ber. I f a bit in the subnet m ask is “0” then the corresponding bit
in the I P address is part of the host I D.
The following exam ple shows a subnet m ask identifying the network num ber (in bold text ) and host
I D of an I P address ( 192.168.1.2 in decim al) .
By convent ion, subnet m asks always consist of a cont inuous sequence of ones beginning from t he
left m ost bit of t he m ask, followed by a continuous sequence of zer os, for a tot al num ber of 32 bit s.
Table 111 Subnet Masks
1ST OCTET:
(192)
2ND
OCTET:
(168)
3RD
OCTET:
(1)
4TH OCTET
(2)
I P Address (Binary) 11000000 10101000 00000001 00000010
Subnet Mask (Binary) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 00000000
Network Num ber 1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1
Host I D 00000010