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Lesson 2 Optimizing subnet designs
Objective Define the methods of optimizing the number of available IP addresses.

Optimizing Subnet designs

When creating an optimal IP addressing scheme for an enterprise network, you will be faced with designing the IP addressing infrastructure using either public IP addresses or private IP addresses. Each method has its advantages and drawbacks.

Public IP addresses

The typical public IP address solution provides an address scheme that is either too large or too small. If the address scheme for a subnet is too large, IP addresses go unused and therefore are wasted. If the address scheme is too small, multiple address ranges and a more complicated routed path are required to achieve the goals of the network design.

Private IP addresses

The typical private address solution circumvents these limitations because you can use a Class A, B, or C network ID that belongs to the group of private networks and subnet them any way you like. However, in order to implement a private addressing scheme on an internal network that needs access to the Internet, you need to implement extra technologies such as Network Address Translation [1].
To solve problems related to IP addressing, you need to learn how to optimize your IP addressing scheme, regardless of whether you use public or private IP addresses.
To optimize available IP addresses, you can:
  1. Divide IP ranges with variable length subnetting
  2. Combine IP ranges by supernetting
  3. Determine the number of subnets and IP addresses used by remote network segments.

Variable length subnetting

Variable length subnetting subdivides an existing network. The subnet mask is adjusted in a hierarchical fashion to allow for a varying number of hosts per subnet.
Variable length subnetting is dependent on routers that support routing protocols, which support variable length subnet masking. For example, RIP 1.0 did not support VLSM. However, RIP 2.0, does support VLSM, using Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP).
Before creating IP addressing designs that incorporate supernetting and CIDR, ensure that all routers support CIDR. Any router that supports RIP for IP 2.0, OSPF, or BGP can be used in a supernetting design.


Supernetting allows a design to combine multiple IP address ranges into a large single range of addresses. For example, supernetting would allow a design to combine eight Class C network addresses into a single IP address range.
Supernetting is the converse of subnetting. When you supernet, you "steal" bits from the network ID in order to create more host IDs per network. For example, imagine that you have been given eight Class C network IDs. You can collapse those eight Class C network IDs into single supernet by taking three bits from the network ID. Now, if you went through the process of bitwise ANDing, you would find that they all AND as local to each other.
To find out more about supernetting, go to the Resources page to view RFC 1519. The next lesson examines how to optimize IP performance on your network.

[1]Network Address Translation:A Network Address Translation (NAT) is designed for IP address conservation. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses, before packets are forwarded to another network.

Border Gateway Protocol (BGP): A routing protocol designed for use between autonomous systems. BGP is especially useful for detecting routing loops.