Figure 278 Network Number And Host Id; Table 138 Ip Address Network Number And Host Id Example - ZyXEL Communications G.SHDSL.bis 4-port Security Gateway P-793H User Manual

Appendix E IP Addresses and Subnetting

Figure 278 Network Number and Host ID

How much of the IP address is the network number and how much is the host ID varies
according to the subnet mask.
Subnet Masks
A subnet mask is used to determine which bits are part of the network number, and which bits
are part of the host ID (using a logical AND operation). The term "subnet" is short for "sub-
network".
A subnet mask has 32 bits. If a bit in the subnet mask is a "1" then the corresponding bit in the
IP address is part of the network number. If a bit in the subnet mask is "0" then the
corresponding bit in the IP address is part of the host ID.
The following example shows a subnet mask identifying the network number (in bold text)
and host ID of an IP address (192.168.1.2 in decimal).

Table 138 IP Address Network Number and Host ID Example

IP Address (Binary)
Subnet Mask (Binary)
Network Number
Host ID
By convention, subnet masks always consist of a continuous sequence of ones beginning from
the leftmost bit of the mask, followed by a continuous sequence of zeros, for a total number of
32 bits.
Subnet masks can be referred to by the size of the network number part (the bits with a "1"
value). For example, an "8-bit mask" means that the first 8 bits of the mask are ones and the
remaining 24 bits are zeroes.
390
1ST 2ND
OCTET: OCTET:
(192) (168)
11000000 10101000
11111111 11111111
11000000 10101000
3RD
4TH OCTET
OCTET:
(2)
(1)
00000001 00000010
11111111 00000000
00000001
00000010
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