ZyXEL ENC User Guide - Page 352
Subnet Masks, Table 195, 1ST OCTET, OCTET, 4TH OCTET
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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 291 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 195 IP Address Network Number and Host ID Example 1ST OCTET: 2ND OCTET: (192) (168) 3RD OCTET: (1) IP Address (Binary) 11000000 10101000 00000001 Subnet Mask (Binary) 11111111 11111111 11111111 Network Number 11000000 10101000 00000001 Host ID 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. 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. 352 ENC User's Guide