So what kind of equipment works at Layer 3? Well by definition that is a router. So a router's job is to look at the destination ip address in a datagram, look that up in its forwarding table to decide where to send it next. Now a collection of routers working together and managed under the same administrative control is called an Autonomous System. And then multiple Autonomous Systems linked together essentially meaning different organizations, different ISPs, different universities and so on, all of those linked together will then in turn form the entire internet. Now unlike the switches which are able to build their bridge tables automatically the forwarding tables have to be set up on routers and there are a number of ways that this can be done. You can do it entirely by hand if you want which is called Static Routing or you can use it Dynamically. So that means the routers exchange information explicitly with each other to learn the topology of your network and to learn what ip subnets exist on the network and how to reach them. So within an autonomous system between your own routers you would use what's called an IGP, an Interior Gateway Protocol, and most common examples of those are OSPF and IS-IS. Between two different autonomous systems then you would use an Exterior Gateway Protocol and the only one really in use these days is called BGP which is the Border Gateway Protocol. So we've looked at Layer 1, 2 and 3 and if you look at how a network is built up you will see that the network is divided into areas which are called domains. So if you have a connection of switches that are connected together that is called a broadcast domain. It's called that because if any one of the devices sends a broadcast then the switches are forced to re-transmit that broadcast to every device on every port in that network. So a broadcast will flood across that area but routers don't forward broadcasts. I mean the broadcast will arrive at the router. The router will if it will process it if it needs to process it if the router if this broadcast is of interest to the router but otherwise it will not forward that broadcast to another port. So routers divide your network up into broadcast domains. On the right hand side of this diagram there's another broadcast domain with switches but there's also a collision domain. A collision domain is where devices at Layer 1 are sharing the same medium and in the olden days this might have been a hub network with ethernet but these days the example you're going to come across is access points. So an access point with a number of devices talking to that access point forms a collision domain because at any one instant in time, only one device on that particular channel on that particular access point is able to transmit and all the others have to be listening. They take it in turns. So there you can see you've got broadcast domains and collision domains and the rule of networking is basically to keep your broadcast domain small and to keep your collision domains even smaller. So let's put that into some network design guidelines. So the guideline we strongly recommend to you, particularly for a campus network environment, is to put no more than 250 hosts on one subnet, meaning in one broadcast domain. So the implication from IPv4 addressing is that your subnet doesn't need to be any larger than a /24 which is enough for 256 ip addresses minus a couple for special uses. What we really mean is 250 devices online at the same time on that network. So you may want to make your address space a bit bigger if for example you've got a wireless network with a lot of churn. So people come in, they pick up an ip address through DHCP and two minutes later they walk away. That address may be reserved to them for a period of time depending on what you've set as your lease time on your DHCP server. So it could be that you need to make your network larger to allow those ip addresses to be allocated so you don't run out of ip addresses but you're still aiming to try and have no more than 250 devices active on one subnet at a particular time. Now in a campus environment our guideline is this you will need at least one subnet per building. So your rule of thumb is: if it's a building it will have its own fiber connection and it will have its own subnet. Now if your building has more than 250 devices in it then you will need more than one subnet because we said a limit of 250 hosts on one subnet. So that might mean you divide your building into floors or you might divide it into wings or you might have one subnet for staff connections and one subnet for computer labs. Or if you've got a really big computer lab you might have multiple subnets for the computer lab or whatever but you aiming to have no more than 250 hosts on a subnet. And you should keep a separate subnet for every building. Now in terms of wireless, all of this still applies but the point about collision domains means really you would prefer to have many access points each serving a smaller number of users or a smaller area rather than one big access point that covers a large area because if it's a big access point and all your users are on the same access point that means they will be sharing the same collision domain and that's not good because it will limit how much throughput you can get. So what you want is you want to have multiple access points and you also want to ensure that those access points don't interfere with each other at Layer 1. Access Points that are close to each other, neighboring access points, should be on non-overlapping radio channels. So for 2.4 gigahertz that would be channel 1, channel 6 or channel 11. Those are non-overlapping channels and a good wireless management system will help you with that but you will probably want to do a proper site survey and help it along to make sure that those channels are non-overlapping as far as possible.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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