ZyXEL NBG6615 User Guide - Page 140
Subnet Masks, Network Size, 1ST OCTET, 2ND OCTET, 4TH OCTET, BINARY, DECIMAL
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Appendix A IP Addresses and Subnetting 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 50 IP Address Network Number and Host ID Example 1ST OCTET: 2ND OCTET: 3RD OCTET: (192) (168) (1) IP Address (Binary) Subnet Mask (Binary) Network Number Host ID 11000000 11111111 11000000 10101000 11111111 10101000 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. 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. Subnet masks are expressed in dotted decimal notation just like IP addresses. The following examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit and 29-bit subnet masks. Table 51 Subnet Masks BINARY 1ST OCTET 8-bit mask 16-bit mask 24-bit mask 29-bit mask 11111111 11111111 11111111 11111111 2ND OCTET 00000000 11111111 11111111 11111111 3RD OCTET 00000000 00000000 11111111 11111111 4TH OCTET 00000000 00000000 00000000 11111000 DECIMAL 255.0.0.0 255.255.0.0 255.255.255.0 255.255.255.248 Network Size The size of the network number determines the maximum number of possible hosts you can have on your network. The larger the number of network number bits, the smaller the number of remaining host ID bits. An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 with a 24-bit subnet mask, for example). An IP address with host IDs of all ones is the broadcast address for that network (192.168.1.255 with a 24-bit subnet mask, for example). NBG6615's User's Guide 140