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Chapter 3. Planning, Implementing, and M... > Create an IP Subnet Scheme

Create an IP Subnet Scheme

Implementing subnets helps control network traffic and enables network administrators to create smaller collision domains. Every node on the same physical ethernet network sees all data packets sent out on the network, which results in multiple collisions and affects network performance. Routers or gateways separate networks into subnets. Subnet masks on each node allow nodes on the same subnetwork to continue communicating with one another and with the routers or gateways they use to send their messages.

Subnet masking enables you to identify the network ID and host (node) ID of an IP address. The following example is a default Class B subnet mask:

10110110.10100101.00110111.01100010 182.165.55.98
11111111.11111111.00000000.00000000 255.255.000.000
---------------------------------------------------
10110110.10100101.00000000.00000000 182.165.000.000
IP Address       : 182.165.55.98
Address Class    : B
Network Address  : 182.165.0.0

Subnet Address   : 182.165.48.0
Subnet Mask      : 255.255.240.0
Subnet bit mask  : 11111111.11111111.11110000.00000000
Subnet Bits      : 20
Host Bits        : 12
Possible Number of Subnets : 16
Hosts per Subnet : 4094

Selected Subnet  : 182.165.0.0/255.255.240.0
Usable Addresses : 4094
     Host range  : 182.165.0.1  to  182.165.15.254
     Broadcast   : 182.165.15.255

To subnet networks further, more bits can be added to the subnet mask for a class of addresses.

The following example is a Class B address using an additional bit subnet mask of 240. Notice that instead of having the single subnet and 65,534 hosts per subnet allowed under the default subnet mask, you can have up to 16 subnets with up to 4,094 hosts per subnet by using a subnet mask of 255.255.240.000 (Table 3.1 shows a sample IP addressing scheme):

10110110.10100101.00110111.01100010 182.165.55.98
11111111.11111111.11110000.00000000 255.255.240.000 Subnet Mask
--------------------------------------------------------
IP Address       : 182.165.55.98
Address Class    : B
Network Address  : 182.165.0.0

Subnet Address   : 182.165.48.0
Subnet Mask      : 255.255.240.0
Subnet bit mask  : 11111111.11111111.11110000.00000000
Subnet Bits      : 20
Host Bits        : 12
Possible Number of Subnets : 16
Hosts per Subnet : 4094

Selected Subnet  : 182.165.0.0/255.255.240.0
Usable Addresses : 4094
     Host range  : 182.165.0.1  to 182.165.15.254
     Broadcast   : 182.165.15.255

Table 3.1. Example of an IP Addressing Scheme
SubnetMaskSubnet SizeHost RangeBroadcast
182.165.0.0255.255.240.04094182.165.0.1 to 182.165.15.254182.165.15.255
182.165.16.0255.255.240.04094182.165.16.1 to 182.165.31.254182.165.31.255
182.165.32.0255.255.240.04094182.165.32.1 to 182.165.47.254182.165.47.255
182.165.48.0255.255.240.04094182.165.48.1 to 182.165.63.254182.165.63.255
182.165.64.0255.255.240.04094182.165.64.1 to 182.165.79.254182.165.79.255
182.165.80.0255.255.240.04094182.165.80.1 to 182.165.95.254182.165.95.255
182.165.96.0255.255.240.04094182.165.96.1 to 182.165.111.254182.165.111.255
182.165.112.0255.255.240.04094182.165.112.1 to 182.165.127.254182.165.127.255
182.165.128.0255.255.240.04094182.165.128.1 to 182.165.143.254182.165.143.255
182.165.144.0255.255.240.04094182.165.144.1 to 182.165.159.254182.165.159.255
182.165.160.0255.255.240.04094182.165.160.1 to 182.165.175.254182.165.175.255
182.165.176.0255.255.240.04094182.165.176.1 to 182.165.191.254182.165.191.255
182.165.192.0255.255.240.04094182.165.192.1 to 182.165.207.254182.165.207.255
182.165.208.0255.255.240.04094182.165.208.1 to 182.165.223.254182.165.223.255
182.165.224.0255.255.240.04094182.165.224.1 to 182.165.239.254182.165.239.255
182.165.240.0255.255.240.04094182.165.240.1 to 182.165.255.254182.165.255.255


When you use standard subnet masks in classful IP addressing schemes, you can plan how many hosts you can support per subnet and how many subnets are available for use. Table 3.2 shows classful IP addressing schemes and uses 255.x.0.0 as the default mask for Class A addresses, 255.255.x.0 as the default mask for Class B class addresses, and 255.255.255.x as the mask for Class C addresses. In these classes, the X is the subnet mask variable in the table’s Subnet Mask column. The table identifies how many subnets ID are supported by each subnet mask and the maximum number of hosts per subnet.

Table 3.2. Subnet Masking for Classful IP Addressing
Subnet MaskNumber of Subnets in Classful RangeNumber of Class A Hosts per SubnetNumber of Class B Hosts per SubnetNumber of Class C Hosts per Subnet
0116,777,21465,534254
12828,388,60632,766126
19244,194,30216,38262
22482,097,1508,19030
240161,048,5744,09414
24832524,2862,0466
25264262,1421,0222
254128131,070510N/A
25525665,534254N/A


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