Subnet Mask - Comtech EF Data Vipersat SLM-5650A User Manual

In the process of subnetting, bits are borrowed from the host ID portion of an IP
address and are then given to the network ID. Then a "Subnet Mask" gets
assigned to the host along with the IP address.
Subnetting is required if the network is segmented.

Subnet Mask

The Subnet Mask is used by the host to determine if a destination IP address is
on the local or on a remote network segment. The table in Figure A-6 shows the
default subnet mask used for each class of IP address.
Address Class Address Class
Class A Class A
Class B Class B
Class C Class C
The IP address and subnet mask work with each other to identify a network
element or device. The subnet mask, like an IP address, contains 4 octets sepa-
rated by a dot (.) and looks similar to an IP address.
The subnet mask determines what bits in the IP address are being used to deter-
mine the network ID by using the Boolean math operator and in a process
called anding. Anding compares each bit value in the IP address with the bit
value in the subnet mask. The result of the anding process determines which
subnet the IP address is on. The Boolean and operator works as follows:
The table shown in Figure A-7 shows the anding of two binary values and the
corresponding decimal equivalents for each of the values.
A p p e n d i x A - N e t w o r k A d d r e s s i n g
Mask Decimal Value Mask Decimal Value
255.0.0.0 255.0.0.0
255.255.0.0 255.255.0.0
255.255.255.0 255.255.255.0
Figure A-6
Default Subnet Masks for IP Classes
0 and 0 = 0
1 and 0 = 0
1 and 1 = 1
CLASS A
CLASS B
CLASS C
Mask Binary Value Mask Binary Value
11111111.00000000.00000000.00000000 11111111.00000000.00000000.00000000
11111111.11111111.00000000.00000000 11111111.11111111.00000000.00000000
11111111.11111111.11111111.00000000 11111111.11111111.11111111.00000000
I P A d d r e s s in g
A-9