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Saturday, November 18, 2006

Subnetting Table

Subnetting Table

Use this table to help you calculate Subnet Masks, available hosts per subnet or number of subnets per class. The table deals with Subnetting of only one octet.

		# of Masked bits	# of Non-masked bits	Bit pattern	# of subnets =2^M	What will the new subnet mask be?	# of hosts per subnet (C Class) =[(2^N)-2]	# of hosts per subnet (B Class) =[(2^N)-2]	# of hosts per subnet (A Class) =[(2^N)-2]
None		0	8	00000000	0	0	254	65,534	~16 Million
1	1	1	7	10000000	2	128	126	32,766	~8 Million
2	2	2	6	11000000	4	192	62	16,382	~4 Million
3	4	3	5	11100000	8	224	30	8,190	~2 Million
4	8	4	4	11110000	16	240	14	4,094	~1 Million
5	16	5	3	11111000	32	248	6	2,046	~520,000
6	32	6	2	11111100	64	252	2	1,022	~260,000
7	64	7	1	11111110	128	254	-	510	~130,000
8	128	8	0	11111111	256	255	-	254	~65,000

M=Number of Masked bits

N=Number of Non-masked bits - Remember that in Class A and B networks you have other octets besides the one you're subnetting. Therefore in a Class A network instead of saying 7 (for example) you must say 7+8+8. In a Class B network instead of saying 5 (for example again, duh!) you must say 5+8. Only in Class C networks does the N value remain the same.

	Starting bits for the first octet	How many networks per Class?	How many hosts per network?	Range of the first octet
Class A	0	126	(2^24)-2=~16 Million	1-126	127=Loopback
Class B	10	2^14=16384	(2^16)-2=~65000	128-191
Class C	110	2^21=~2 Million	(2^8)-2=254	192-223
Class D	1110	-	-	224-239
Class E	1111	-	-	240-255