IP Address Question

[rgharper]

I think the article did not mean that you need to be picky about what private range you take - I think it meant that the formerly common practice of randomly picking any IP address range to be used internally instead of using a private address range instead should be avoided for the obvious reasons.

Many network technicians used to just pick an address range at random and use it inside their router. Let's say that you decided to set up a new network and decided to randomly pick a set of addresses (say maybe 207.138.0.x - 207.138.128.x) as your "internal" address ranges. That would work fine, and NAT would indeed keep the outside world from seeing your address range ... but your choice of address range would cut you off from about 40% of the available Microsoft Web resources since those addresses would be resolved internally instead of externally.

Picking random addresses usually worked in the 60's, 70's and 80's but from about the mid-90's on the chance that your "private" address range would intersect with a public range started growing substantially. Nowadays it's almost impossible to pick a random set of addresses and not have it assigned to - or worse, actually going - somewhere.

[Pinnacle]

Thanks to each of you for your answers. I assumed that if I used 207.138.128.10 for my private IP address that the router would differentiate that from 207.138.128.10 the public IP address because each IP address would be on opposite sides of the router. While this logic is probably possible, I can understand why it would be simpler to just have established private IP address ranges for everyone to use so that this is not an issue. Thanks again.

[rgharper]

Thanks to each of you for your answers. I assumed that if I used 207.138.128.10 for my private IP address that the router would differentiate that from 207.138.128.10 the public IP address because each IP address would be on opposite sides of the router. While this logic is probably possible, I can understand why it would be simpler to just have established private IP address ranges for everyone to use so that this is not an issue. Thanks again.

The problem is that your router will never make an external inquiry for 207.138.x.x addresses because it "knows" it's an internal address. Likewise, any internal DNS servers would also "know" that anything in the 207.138.x.x range should be an internal address and won't go looking outside for the resources that might exist. That's why the private address blocks were defined and why good network design always uses them internally instead of just taking any block of addresses and using them instead.

If you did manage to program your router to work around this somehow, and the time and effort it would take would be pretty monumental, you'd still have conflicts when an address inside directly matches an address outside. Your network would favor the internal address over the external address so you'd still be unable to access external resources in the case of a matched address conflict.

[slgrieb]

Not all routers by any means will work with the address ranges of all 3 classes. You can't take take a Linksys BEFSR 41 and set its internal address to a Class B address.