Appendix |
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Netmask |
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The partition scheme that separates the different address classes is used to identify a |
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netmask attached to the IP address. A netmask is a |
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bination (using an AND operator) with an IP address, forms the network address. The |
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netmasks for Class A, B and C are for instance 255.0.0.0, 255.255.0.0 and 255.255.255.0. |
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The address 192.168.170.237 is a Class C IP address; its network portion is 192.168.170. |
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Combining it with the Class C netmask using an AND operator as shown in this example |
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leaves only the network portion of the address: |
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11000000 10101000 10101010 11101101 (192.168.170.237) |
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combined with: |
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11111111 11111111 11111111 00000000 (255.255.255.0) | EN |
resulting in: |
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11000000 10101000 10101010 00000000 (192.168.170.0) |
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An alternative to dotted decimal representation is to express the netmask as the quantity |
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of ones from left to right. This number is added to the IP address after a slash (/) as '/n'. |
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In this example, the address 192.168.170.237/24 could be written. This indicates that the |
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netmask is 24 ones followed by eight zeroes. |
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Subnet Addressing |
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You can see that even with a Class C address, there are a large number of hosts per net- |
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work. It would be a very inefficient use of the amount of IP addresses to supply every |
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LAN with so many IP addresses. A smaller office LANs does not have that many devices. |
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A more efficient technique is known as subnet addressing. |
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Subnet addressing allows us to split one IP network address into smaller multiple physi- |
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cal networks known as subnetworks. Some of the node numbers are used as a subnet |
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number instead. A Class B address gives us 16 bits of node numbers translating to 64,000 |
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nodes. Most organizations do not use 64,000 nodes, so there are free bits that can be |
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reassigned. Subnet addressing makes use of those bits that are free. |
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A Class B address can be effectively translated into multiple Class C addresses. For |
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example, the IP address of 172.16.97.235 would be interpreted as IP network address |
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172.16, subnet number 97, and node number 235. In addition to extending the number |
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of addresses available, subnet addressing provides other benefits. Subnet addressing |
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allows a network manager to construct an address scheme for the network by using |
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different subnets for other geographical locations in the network or for other depart- |
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ments in the organization. |
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115
