ZyXEL SBG3300-N000 User Guide - Page 386

Appendix B 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 197 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 128 Subnet Masks IP Address (Binary) Subnet Mask (Binary) Network Number Host ID 1ST OCTET: 2ND OCTET: (192) (168) 11000000 10101000 11111111 11111111 11000000 10101000 3RD OCTET: (1) 00000001 11111111 00000001 4TH OCTET (2) 00000010 00000000 00000010 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. 386 SBG3300-N Series User's Guide