ZyXEL Communications STB-1001S Podręcznik Użytkownika Pobierz pdf (Strona 116)

Appendix B IP Addresses and Subnetting

STB-1001S User’s Guide

116

Figure 81 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).

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.

Table 25 IP Address Network Number and Host ID Example

1ST OCTET:

(192)

2ND

OCTET:

(168)

3RD

OCTET:

(1)

4TH OCTET

(2)

IP Address (Binary) 11000000 10101000 00000001 00000010

Subnet Mask (Binary) 11111111 11111111 11111111 00000000

Network Number 11000000 10101000 00000001

Host ID 00000010

1 2 ... 111 112 113 114 115 116 117 118 119 120 121 ... 136 137

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