ZyXEL Communications NBG-417N manual Subnet Masks, 206, Subnet Mask Identifying Network Number

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 115 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 69 Subnet Mask - Identifying Network Number

	1ST	2ND	3RD	4TH
	OCTET:	OCTET:	OCTET:	OCTET
	(192)	(168)	(1)	(2)
IP Address (Binary)	11000000	10101000	00000001	00000010
Subnet Mask (Binary)	11111111	11111111	11111111	00000000

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NBG-417N User’s Guide