Table 92 Allowed Ip Address Range By Class; Table 93 "Natural" Masks; Subnet Masks - ZyXEL Communications VSG-1200 User Manual

VSG-1200 User's Guide
Since the first octet of a class "A" IP address must contain a "0", the first octet of a class "A"
address can have a value of 0 to 127.
Similarly the first octet of a class "B" must begin with "10", therefore the first octet of a class
"B" address has a valid range of 128 to 191. The first octet of a class "C" address begins with
"110", and therefore has a range of 192 to 223.

Table 92 Allowed IP Address Range By Class

CLASS
Class A
Class B
Class C
Class D

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 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 as IP addresses are. The "natural"
masks for class A, B and C IP addresses are as follows.
Table 93
CLASS
A
B
C
Subnetting
With subnetting, the class arrangement of an IP address is ignored. For example, a class C
address no longer has to have 24 bits of network number and 8 bits of host ID. With
subnetting, some of the host ID bits are converted into network number bits. By convention,
subnet masks always consist of a continuous sequence of ones beginning from the left most bit
of the mask, followed by a continuous sequence of zeros, for a total number of 32 bits.
Since the mask is always a continuous number of ones beginning from the left, followed by a
continuous number of zeros for the remainder of the 32 bit mask, you can simply specify the
number of ones instead of writing the value of each octet. This is usually specified by writing
a "/" followed by the number of bits in the mask after the address.
250
ALLOWED RANGE OF FIRST OCTET
(BINARY)
00000000 to 01111111
10000000 to 10111111
11000000 to 11011111
11100000 to 11101111
"Natural" Masks
NATURAL MASK
255.0.0.0
255.255.0.0
255.255.255.0
ALLOWED RANGE OF FIRST OCTET
(DECIMAL)
0 to 127
128 to 191
192 to 223
224 to 239
Appendix H IP Subnetting