David sets up his new home network so that his whole family can share the internet and his inkjet printer. It is all wired using Ethernet cables. He has one network home router that everything including the cable mode is plugged into. Everything is working fine. Then David decides to add wireless to his network so he can use the wireless capability of his laptop. He buys a wireless router at the store, brings it home, plugs it into his home network router, adds his laptop to the wireless network, and he can browse the internet from his laptop. Everything looks like it is working until he tries to print from the laptop. He then discovers that the laptop can’t print to the printer.
What did David do wrong? Basically he used two routers in his home. If you want to keep life simple, don't use more than one router on your home network. It would have been simpler if David had simply unplugged his old wired home router and replaced it with his new wireless router.
If you find that explanation lacking, and wonder why having two routers is bad, but you don’t want to get bogged down in understanding networking technology, or you would like to be able to explain this issue to others, I offer the following analogy.
1. Think of the network as being like a stream of water. When you enter a name like www.hp.com into your browser, you are sending a request “upstream”, like a salmon traveling upstream to spawn. When your requests fulfill their mission, the results of their endeavors flow back downstream to you and show up in your browser as a web page.
2. Routers have one single upstream port (which is usually labeled “WAN”); this is what connects to the internet (i.e., cable modem or DSL modem). All the other connections are “downstream ports”; these are what you plug your computer into. So the internet is upstream and your home is downstream.
3. Think of your router being like a dam on the stream. It separates upstream from downstream. It is specially designed to let the www.hp.com requests (the adult salmon) travel upstream. It also lets the little offspring of the salmon float back down to your browser window. The key thing is that it doesn’t let adult salmon swim downstream. In other words, it does not let requests from people outside of you home into your home. This mechanism is what provides a level of security against data coming from the internet. People outside your home can’t request information from inside your home.
So now David’s problem, having two routers, should be fairly easy to see and understand. When he added a wireless router, he was adding a new dam inside his home network stream and this new dam was clogging things up. More specifically the printer was upstream while the laptop was downstream. The printer was upstream because it was connected to the old wired home router. David’s laptop was wirelessly connected to new router, and so was downstream. Being downstream, the laptop could send its requests swimming upstream to the internet, but the printer could not send its requests downstream because they got blocked by the dam. Without requests coming back down from the printer, the laptop was prevented from discovering the printers IP address and so couldn’t print to it.
Sorry if you find my salmon analogy a bit too graphic, but thinking of networks as having upstream and downstream parts is very common and useful:
So what does this have to do with “Multiple Subnets”? The upstream and downstream parts are each two different sub-networks, or subnets for short. If everything in your home is downstream together, nothing upstream from anything else, you just have the one subnet which is your home network. Anything else is multiple subnets. So someone might ask if you have multiple subnets; now you know that this is just another way of asking if you have multiple routers in your home network. Another uber-geek term someone might throw at you is to ask if you are “double NATed”. NAT is the name of the damming mechanism in the router, so this is just another techie way of asking if you have two routers.