Technical Library

Subnetting - What, When, and How

Sample Subnetting Topology

How to Subnet Your Network

The most important thing to remember when using a 24 bit subnet of our Class A 38 network is that it is *NOT* a real Class C network. Due to this fact it is easier to subnet than a Class C.

Remember that the subnet portion of an IP address cannot be all 0s or all 1's. Thus, when a Class C network is subnetted you cannot use a 1 bit subnet mask, since the subnet portion will either be 1 or 0 (which is all 1s and all 0s respectively). If you use a 2 bit subnet mask, you lose the 00 and 11 subnet and only the 01 and 10 subnets are usable.

The network portion of a Class A address is only 8 bits; the following 16 bits are part of the subnet portion of the address. If you add an additional bit to the subnet mask, then the number of subnet bits is 17 not 1. The subnet portion of the address is not 0 or 1 but xxxxxxxx xxxxxxxx 0 or xxxxxxx xxxxxxxx 1. This also follows for any number of subnet bits that are added. The only addressing space that is lost is the all 0s and all 1s hosts as these are still reserved for the network address, and broadcast address.

The following are the numbering schemes for the most common Subnet configurations for subnetting a 38.x.y.0 address:

25 Bit

Subnetwork Bits:         25
Number of Subnets:        2
Hosts per Subnet:       126
Total Number of Hosts:  252
Netmask:                255.255.255.128

Subnet	
Bits		Network		Range		Valid Hosts
------		-------		-----		-----------
0 		  0  		0-127		1-126
1 		128		128-255		129-254

26 Bit

Subnetwork Bits:         26
Number of Subnets:        4
Hosts per Subnet:        62
Total Number of Hosts:  248
Netmask:                255.255.255.192

Subnet	
Bits		Network		Range		Valid Hosts
------		-------		-----		-----------
0 0		  0 		0-63		1-62
0 1		 64		64-127		65-126
1 0		128		128-191		129-190
1 1		192 		192-255		193-254

27 Bit

Subnetwork Bits:	 27
Number of Subnets:	  8
Hosts per Subnet:	 30
Total Number of Hosts:	240
Netmask:		255.255.255.224

Subnet  
Bits    	Network         Range           Valid Hosts
------  	-------         -----           -----------
0 0 0		  0 		0-31		1-30
0 0 1	 	 32		32-63		33-62
0 1 0	 	 64		64-95		65-94
0 1 1	 	 96		96-127		97-126
1 0 0		128		128-159		129-158
1 0 1		160		160-191		161-190
1 1 0		192		192-223		193-222
1 1 1		224 		224-255		225-254

28 Bit

Subnetwork Bits:         28
Number of Subnets:       16 
Hosts per Subnet:        14
Total Number of Hosts:  224 
Netmask:                255.255.255.240

Subnet
Bits    	Network         Range           Valid Hosts
------  	-------         -----           -----------
0 0 0 0		  0 		0-15		1-14
0 0 0 1		 16		16-31		17-30
0 0 1 0		 32		32-47		33-46
0 0 1 1		 48		48-63		49-62
0 1 0 0		 64		64-79		65-78
0 1 0 1		 80		80-95		81-94
0 1 1 0		 96		96-111		97-110
0 1 1 1		112		112-127		113-126
1 0 0 0		128		128-143		129-142
1 0 0 1		144		144-159		145-158
1 0 1 0		160		160-175		161-174
1 0 1 1		176		176-191		177-190
1 1 0 0		192		192-207		193-206
1 1 0 1 	208		208-223		209-222
1 1 1 0		224		224-239		225-238
1 1 1 1		240 		240-255		241-254

29 Bit

Subnetwork Bits:         29
Number of Subnets:       32 
Hosts per Subnet:         6
Total Number of Hosts:  192
Netmask:                255.255.255.248

30 Bit

Subnetwork Bits:         30
Number of Subnets:       64 
Hosts per Subnet:         2
Total Number of Hosts:  128
Netmask:                255.255.255.252