Routing Question

Rearden

Hey there, I have a question about routing. At first, I thought since routers talk to each other that every router on the Internet must know about every other router. Then I realized this would be impossibly complex. So now, I have learned, or at least I think I have learned that routers only know about the routers attached directly to them? Their routing tables consist of where to send a packet based on the subnet it is on. Is that correct? My final question is, do routers know about every subnet or can a home router for example say "everything that is not 192.168.0.0 255.255.0.0 goes out X interface?


Thanks,
Matt

EdTheLad

Routers on the internet must have routes to all possible networks(this is without taking route manipulation into account),to reduce the routing table summarization is used,but still the routing table can be hugh.More that 100,000 routes.BGP is a powerful routing protocol that is used for the internet,a routing running BGP can download the complete internet routing table from its neighbor,this is the reason why a BGP router needs to be a powerful platform containing alot of ram.

agustinchernitsky

Regarding your last question, you have your default gateway to do that. Normally, in your "home router" and in your PCs you have a setting in your TCP/IP that says "default gateway".

What it does is quite simple: When you send a paquet whose destination is not on the local network, it sends it to your gateway (router). If the router doesn't know where to send it, it sends it to its default gateway (ISP). The ISP uses BGP, RIP, RIPv2, and many others to route the paquet in the very big Internet.

How does your PC know when to forward a paquet to your gateway? It uses your IP in combination with your subnet mask: 192.168.0.10 / 255.255.255.0 means that your local network is 192.168.0.0. If you send paquets to 192.168.1.10, then, your PC sends the paquet to the router...

Hope it helps.