ZyXEL NBG4604 User Guide - Page 204

Subnet Masks

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Appendix A IP Addresses and Subnetting The following figure shows an example IP address in which the first three octets (192.168.1) are the network number, and the fourth octet (16) is the host ID. Figure 127 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 75 Subnet Mask - Identifying Network Number 1ST OCTET: 2ND OCTET: 3RD OCTET: 4TH OCTET IP Address (Binary) Subnet Mask (Binary) (192) (168) (1) (2) 11000000 10101000 00000001 00000010 11111111 11111111 11111111 00000000 204 NBG4604 User's Guide

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Appendix A IP Addresses and Subnetting
NBG4604 User’s Guide
204
The following figure shows an example IP address in which the first three octets
(192.168.1) are the network number, and the fourth octet (16) is the host ID.
Figure 127
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 75
Subnet Mask - Identifying Network Number
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