ZyXEL MI-7248 User Guide - Page 280
Subnet Masks
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Appendix A IP Addresses and Subnetting Table 115 Classes of IP Addresses (continued) IP ADDRESS OCTET 1 OCTET 2 OCTET 3 Class B Network number Network number Host ID Class C Network number Network number Network number OCTET 4 Host ID Host ID An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 for example). An IP address with host IDs of all ones is the broadcast address for that network (192.168.1.255 for example). Therefore, to determine the total number of hosts allowed in a network, deduct two as shown next: • A class C address (1 host octet: 8 host bits) can have 28 - 2, or 254 hosts. • A class B address (2 host octets: 16 host bits) can have 216 - 2, or 65534 hosts. A class A address (3 host octets: 24 host bits) can have 224 - 2 hosts, or approximately 16 million hosts. IP Address Classes and Network ID The value of the first octet of an IP address determines the class of an address. • Class A addresses have a 0 in the leftmost bit. • Class B addresses have a 1 in the leftmost bit and a 0 in the next leftmost bit. • Class C addresses start with 1 1 0 in the first three leftmost bits. • Class D addresses begin with 1 1 1 0. Class D addresses are used for multicasting, which is used to send information to groups of computers. • There is also a class E. It is reserved for future use. The following table shows the allowed ranges for the first octet of each class. This range determines the number of subnets you can have in a network. Table 116 Allowed IP Address Range By Class CLASS ALLOWED RANGE OF FIRST OCTET (BINARY) ALLOWED RANGE OF FIRST OCTET (DECIMAL) Class A 00000000 to 01111111 0 to 127 Class B 10000000 to 10111111 128 to 191 Class C 11000000 to 11011111 192 to 223 Class D 11100000 to 11101111 224 to 239 Class E 11110000 to 11111111 (reserved) 240 to 255 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). 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. 280 MS-7206 User's Guide