One of the problems I've seen repeatedly over the years is improper allocation of IP network space. Whether the trouble is the use of public IP addresses that are not rightfully owned, or just a lack of understanding of the benefits of proper IP allocation, a poorly designed IP network can cause daily headaches and increased troubleshooting time. And because a poorly designed IP scheme can take months to rectify, the problem usually gets worse over time because no one wants to take on the task of renumbering the entire network.
There are some simple rules that can help when designing a network using IP address space. Here is the first one:
When allocating IP address space, endeavor to allocate a block that can be referenced with a single access-list entry.
Following this simple rule will make everything from server allocations to global network design much simpler.
Assume for a minute that you've been requested to create a network for a new server farm. The powers that be insist there will only ever be 30 servers in the farm. You, being the bright admin, ask what kind of servers they will be. The response is that they will all be Oracle servers.
The average admin might be quick to say that a /27 (255.255.255.224) network would be in order, as it provides 32 host addresses. Two of these are used for network and broadcast addresses, however, leaving no room for growth. This leads us to our next rule:
Always allocate more IP address space than is originally requested. ...