ZyXEL P-660HW-T1 v2 User Guide - Page 298
Subnet Masks
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P-660HW-T v2 User's Guide 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 131 Allowed IP Address Range By Class CLASS ALLOWED RANGE OF FIRST OCTET (BINARY) Class A Class B Class C Class D Class E (reserved) 00000000 to 01111111 10000000 to 10111111 11000000 to 11011111 11100000 to 11101111 11110000 to 11111111 ALLOWED RANGE OF FIRST OCTET (DECIMAL) 0 to 127 128 to 191 192 to 223 224 to 239 240 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. Subnet masks are expressed in dotted decimal notation just like IP addresses. The "natural" masks for class A, B and C IP addresses are as follows. Table 132 "Natural" Masks CLASS A NATURAL MASK 255.0.0.0 298 Appendix F IP Subnetting