Omega Engineering i.BTX-M manual Appendix C IP Netmask, Class B, Class C

Appendix C IP Netmask

IP Netmask or Subnet Mask is a 32-bit pattern of ones and zeros used to determine network portion of an IP address from the host portion of the IP address. Subnet mask is a network ID that is created by borrowing bits from host portion of IP address and using them as part of a network ID. The table below shows a default subnet mask for address Classes A, B, and C. Each bit that is set to "1" in the subnet mask corresponds to the bit in the IP address that is to be used as the network ID. Each bit that is set to "0" in the subnet mask corresponds to a bit in the IP address that is to be used as the host ID.

Address Class	Mask Binary Value			Mask Decimal Value
				or Dotted Notation
Class A	11111111 00000000	00000000	00000000	255.0.0.0
Class B	11111111 11111111	00000000	00000000	255.255.0.0
Class C	11111111 11111111	11111111	00000000	255.255.255.0

If your network requires more network ID’s, you can extend the default subnet mask to include additional bits from the host ID. This allows for additional network ID’s within the network. The table below shows some examples of subnet masks and bits moved from the hosts ID to create a new subnet.

Mask Dotted Notation	Mask Binary			Mask Bits
	Class A
255.0.0.0 (Default)	11111111 00000000	00000000	00000000	0
255.192.0.0	11111111 11000000	00000000	00000000	2
255.224.0.0	11111111 11100000	00000000	00000000	3
255.240.0.0	11111111 11110000	00000000	00000000	4
255.248.0.0	11111111 11111000	00000000	00000000	5
255.252.0.0	11111111 11111100	00000000	00000000	6
255.254.0.0	11111111 11111110	00000000	00000000	7
255.255.0.0	11111111 11111111	00000000	00000000	8
255.255.128.0	11111111 11111111	10000000	00000000	9
255.255.192.0.0	11111111 11111111	11000000	00000000	10
……………	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			.
255.255.255.252	11111111 11111111	11111111	11111100	22
	Class B
255.255.0.0 (Default)	11111111 11111111	00000000	00000000	0
255.255.192.0	11111111 11111111	11000000	00000000	2
……………	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			.
255.255.255.252	11111111 11111111	11111111	11111100	14
	Class C
255.255.255.0 (Default)	11111111 11111111	11111111	00000000	0
255.255.255.192	11111111 11111111	11111111	11000000	2
………………….	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			.
255.255.255.254	11111111 11111111	11111111	11111100	6

To determine the number of valid hosts ID’s remaining after subnetting, use the following equation: 2n – 2, where n is the number of octet digits left after the subnet mask.

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