Default gateway question please help

[AY786]

Ive got this question from college which i need help on. Below is the question. Please read it and then select one of the following options on the poll please if you know the answer, thank you.

Your IP address is 128.0.1.2. You wish to contact a host at 130.120.10.2. Two routers reside between you and the destination. Their IP addresses are 128.0.1.1 and 130.120.1.1. What would be your default gateway?

 

[AMD[H]unter]

The default gateway is 128.0.1.1

 

[AY786]

could you explain why please

 

[atomiser]

the default gateway is the ip address of the router attached to your subnet - in this case 128.0.0.1

 

[RavinDJ]

Damn... I know that it's 128.0.1.1, but I can't explain why. I understand a lot of networking, but I never know WHY something happens or WHY it's true. The sad part is... I actually did take a Computer Networking course in college.

Any ideas where I could re-learn some of the stuff? Or, actually learn it for the first time, the proper way?

Thanks!

P.S. It's 128.0.1.1 because it's where all your outgoing packets have to go in order to be passed onto the rest of the network (that's my crappy answer)

 

[0ldman]

you have to have a local gateway on the same subnet and a router with both subnets.

TCP/IP cannot communicate with devices outside its own subnet without a gateway device to route the packets.

I've dealt with this personally with an ISP that tried to use non-standard (read non-working) routing.

 

[thedude42]

My question is: was there more to the question, something like "assume the usage of default routing."

There are a number of various methods of routing, and default routing is just one of them. However, default routing is the common standard, if you look at standard texts such as "TCP/IP Illustrated" you see that default routing is mentioned as just one of many ways used for getting packets tot he right network.

Routing advertisements used to update a host's routing table are one of the things my networking studies have emphasized simply because it is what routers use to talk to eachother with the various routing protocols they impliment. In UNIX labs each host machine is often configured to use advertisements to bring this point home, rather than using DHCP and default routing, but the majority of host machines (regular workstations) don't employ this method.

The short of it is that TCP/IP has a bunch of routing protocols, but the majority of host machines are configured to simply use default routing and let the routers handle the dynamic updating of routes between networks and various hosts. Thus the workstation can focus it's MUX/DEMUX work on the higher layer protocols, and let the routers focus on the IP routing. Each machine has a routing table, but the common situation (ethernet and its variations) is that the table makes only 3 decisions: is it localhost, local net, or anywhere else? The term "default routing" comes from this idea, in that if the answer isn't one of the first 2 options, then there is a "default route" that all the traffic takes.

The situation blurs a bit when you start talking about VPN's, but for all intents and purposes, you're just adding subsequent "local net" interfaces to ask about before you get to the default route.

 

[Malk-a-mite]

Damn... I know that it's 128.0.1.1, but I can't explain why.

I had a professor that every class would ask:
"Where does the packet go next and why?"

After a couple of weeks the class used to be able to answer in unison.

 

[AY786]

Thanks for the response guys, ive actually not got the answer but ive understood it now as well. This is actually a course i am doing called N+ wonder if any of you have heard of it or done this