CCNA - Nat

Author

Nat

meberer

2004-03-27, 7:04 pm

Does anyone know the difference between the four different address types in NAT?
inside local, inside global, outside local and outside global.

I seems a little bit confusing to me.

Thank you

dmaftei

2004-03-27, 10:53 pm

Here's a way to think of it. You have your internal network, a NAT router, and the Internet.

- inside is what's on your side of the NAT router.
- outside is what's beyond the NAT router.
- local is your stuff, i.e. your IP addresses
- global is stuff that's not yours, i.e. IP addresses that are not under your control.

Now, this things are relevant for your NAT router. You've got two cases here:

- traffic going from your network to the Internet. When the NAT router receives a packet, the source address is inside local, the destination address is outside local. The NAT router performs translation(s), and when it transmits the packet the source address is inside global, the destination address is outside global.

- traffic coming from the Internet to your network. When the NAT router receives a packet, the source address is outside global, the destination address is inside global. The NAT router performs translation(s), and when it transmits the packet the source address is outside local, the destination address is inside local.

One thing to realize here is that local/global may be the same. In the most common case the inside address is translated (inside local and inside global are different), while the outside address is not (outside local and outside global are the same). For instance, if you (192.168.0.2) ping www.examnotes.net (204.127.202.19), then:
- inside local is 192.168.0.2
- inside global is a.b.c.d (given to you by your provider)
- outside local is 204.127.202.19
- outside global is 204.127.202.19

There are also the cases when the outside gets translated and the inside is the same, and when both outside and inside are translated, but I don't remember the purpose of these...

meberer

2004-03-28, 2:18 pm

Thank you for your reply.
The only point I havn't understood completly is the difference between outside global and outside local.
I know that global is the internet and local is e.g. the enterprise network. Und outside is my destination host. That means the destination host in the Internet is outside global. And in some cases this address is forwarded via NAT to a host in the enterprise network.

Can somebody check if this is right?

thank you

dmaftei

2004-03-28, 5:20 pm

I guess your question is when outside local and outside global are different. Say that for some reason (which eludes me) you decide to map www.examnotes.net (204.127.202.19) to one of your private addresses, e.g. 192.168.0.99. You go through the trouble of putting this in your DNS. You also configure your NAT appropriately. Then you ping www.examnotes.net. Your ping reaches your NAT router as:
- source 192.168.0.2 (inside local)
- destination 192.168.0.99 (outside local)
NAT translates to:
- source a.b.c.d (inside global)
- destination 204.127.202.19 (outside global)

Again, I don't remember why this is done, but that's how it works.

meberer

2004-03-30, 11:12 am

Thank you, I think I've understood it now. But I share your opinion. It doesn't really make any sense.

Greetings

dmaftei

2004-03-30, 1:56 pm

quote:
Originally posted by meberer
But I share your opinion. It doesn't really make any sense.

Well, I didn't say it doesn't make sense. I'm sure there are good reasons to do it; it's just that I don't remember them...

meberer

2004-03-31, 3:52 pm

Ok, Ok,
I don't want to inplay something to you.
But thank you for your help.

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