Hi I have this doubt for quite some time.Suppose i got a leased line from an ISP,obviously im going to assign the valid IP from the isp to the serial wan port.If suppose i use a 1700 series router and for the ethernet port if i assign a valid ip address (for which i dont pay)which is already in use by somebody else, and also the NAT feature is also disabled, surely the router willsend a route for the ethernet ip address to the internet.Will it not mesh up the internet.
In that case how this is prevented in real world.Please clarify.Thks.
The router will not, by default, send a route advertisement to your ISP. You would need to configure a routing protocol for that to happen. In the real world, if you only have 1 connection to the Internet you typically do not run any routing protocols with the ISP. You use static routes.
The Internet uses BGP for routing so you would need to run BGP on your router and the ISP would need to configure their router to be a BGP peer with you. It is kind of a built in safety net to keep incorrectly configured gear from interfering with the operation of the Internet.
NAT is a tool to change IP network addresses. It can be configured inbound or outbound, depening on the need.
In most cases with a single connection to an ISP, you are assigned a range of addresses from the ISP to represent your network on the Internet. Depeding on the size of your network behind your router and the number of addresses you have been allocated by your ISP, you might not have to run NAT. It sounds like in your case that you would want to run NAT to translate your internal addresses into the space provided by the ISP.
NAT save you from having to re-address your network to match the address space assigned by the ISP.
I think that any of the "standard" access routers do (by standard I mean a non-unique DSL-only router or something). However, when you get down to the lower platforms (such as the 1720), you need, as a minimum, the IP Plus feature pack instead of just the standard IP pack. So plan your memory carefully because the plus pack requires more.