Switch vs Router

[Stormspark]

Switches nowadays do some work of routers as well (layer IP stuff)

Yes, but if you just go out and get something like an 8 port gigabit switch, it will not do the "connect a bunch of computers to the internet through my cable modem" like a router will - going by what the average person looking for a "router" would want. I was intentionally using a simplistic description of the difference between a switch and a router. In reality, there are only hubs (dumb devices that echo everything to every port - layer 1) to switches of various types. Simple switches only use an ARP table to send data to only the correct port rather than all ports (layer 2). More advanced switches do layer 3 stuff as well (layer 3 is what a consumer "router" that does network address translation would be).

Most "switches" unless you are talking expensive managed switches would be layer 2, while consumer "routers" would be considered layer 3. In reality, they are both technically switches, just one has more features than the other.

You could very easily nuke off a network that only has "simple" layer 2 switches, by connecting cables so as to form a loop between them. Because broadcast traffic goes to ALL ports even on a switch. On a hub, all traffic goes to all ports, on a switch only broadcast traffic does. Connecting layer 2 switches together in a loop will cause a broadcast storm and effectively nuke off the network until the loop is removed. More expensive managed switches implement spanning tree protocol, so you can connect switches together in loops for redundancy without causing broadcast storms. But this is more advanced stuff that most people looking to buy a "router" would never encounter.

[Theodon]

By the way. In order for you to use a switch you need to have more than one IP-address

Why would you need more than 1 IP address to use a switch? Switches don't even use layer 3 addressing unless it's a layer 3 switch.

[Vash The Stampede]

Why would you need more than 1 IP address to use a switch? Switches don't even use layer 3 addressing unless it's a layer 3 switch.

Every device needs its own IP address but that shouldn't be a problem for anyone. DHCP usually takes care of that or you could just give your device it's own IP.

[Stormspark]

Why would you need more than 1 IP address to use a switch? Switches don't even use layer 3 addressing unless it's a layer 3 switch.

They mean, to connect all your computers to the internet with JUST a switch (meaning no router doing NAT), you would need a separate internet-facing IP from your ISP for each computer. With this type of setup, each computer would have its own internet IP. In most people's setups, each computer has its own LOCAL IP in a reserved range that isn't used on the internet. 192.168.x.x, 172.16.x.x, and 10.x.x.x. With ONE internet IP, and a router doing network address translation so that all of the computers can share that one internet IP.

Of course each computer needs its own IP address, at least locally.

[Theodon]

They mean, to connect all your computers to the internet with JUST a switch (meaning no router doing NAT), you would need a separate internet-facing IP from your ISP for each computer. With this type of setup, each computer would have its own internet IP. In most people's setups, each computer has its own LOCAL IP in a reserved range that isn't used on the internet. 192.168.x.x, 172.16.x.x, and 10.x.x.x. With ONE internet IP, and a router doing network address translation so that all of the computers can share that one internet IP.

Of course each computer needs its own IP address, at least locally.

I get that, but I just don't understand how only having one IP would mean you can't use a switch. You can plug a PC in to a switch and it would still work fine. It wouldn't be able to do much beyond telnet in to the switch though. It would be far more accurate to say that you need more than 1 device to really need a switch, but even then you usually have 4 ethernet interfaces on the combi-devices you get from ISPs anyway so you wouldn't need a switch unless you've got a home server on the go.

Nothing is stopping you from doing it, but there would be no point.

[Stormspark]

I get that, but I just don't understand how only having one IP would mean you can't use a switch. You can plug a PC in to a switch and it would still work fine. It wouldn't be able to do much beyond telnet in to the switch though. It would be far more accurate to say that you need more than 1 device to really need a switch, but even then you usually have 4 ethernet interfaces on the combi-devices you get from ISPs anyway so you wouldn't need a switch unless you've got a home server on the go.

Nothing is stopping you from doing it, but there would be no point.

Actually, some "cable/DSL modems" are in fact routers, with only one port. And if you hook several PC's up to the modem via a switch, it will do the NAT for you. YMMV though, not all cable/DSL modems do this or have this functionality, and sometimes devices that do have this functionality do not have it advertised. When I used to work for Verizon years ago, I did talk to one guy that hooked everything up like this and thought the switch was the router. It turned out that his DSL modem was acting as the router for him, even though it wasn't advertised with that capability. My SB6141 cable modem has this functionality also, even though it's not advertised. I don't use it though, instead I have a custom Linux machine acting as my router.

Of course, I had another family that did this, in a DHCP area (as opposed to a PPPOE area), and pulled multiple IP's (because the service didn't stop them from doing it and the modem in that case was NOT a router), and actually had their account suspended for it. Because each customer was only allowed one IP, and instead of designing the equipment to only give them one IP, they didn't bother with that part and just passed the blame onto the customer if it was exceeded. I think they put it in the terms of use or something. They didn't even know they were doing anything wrong, they went out and bought a cheap 4 port switch and thought they got a good deal on a router.

[Cyanotical]

this thread....wow........ it almost needs a disclaimer

okay, a switch is a layer 2 asic device that forwards packets based on MAC address, ( a layer 3 switch is a router using switching hardware)

a switch is dumb and will not care about IP addresses, it will simply forward traffic based on MAC, which means it only talks to things directly connected to itself, so ignore people who say they can track you by MAC address


a router is a layer 3 device that allows traffic to cross networks, and i need to clear up what a network means, a network is a bunch of hosts that exist in the same layer 3 broadcast domain. it has little to do with how they are physically connected

a broadcast domain is the range of IP's that you can talk to without needing to use a router, a broadcast domain is determined by the subnet mask:

to boil this down and skip a bunch, the subnet mask defines the part of the IP address that defines the network:

255.255.255.0 = net.net.net.host

that means that in this example:

192.168.1.100

the bold parts are the network, and the not bold are hosts
hosts can talk to each other, but if they need to use a router to talk to anyone with a different network address

a router can talk to multiple networks and can send traffic from one network to another

so anytime you need to send data to a host that is in the same network as you, you only need a switch, but if you need to talk to another network, you need a router

a DHCP server is not magic, they have to be told what addresses to hand out

you need multiple IP addresses to use a switch instead of a router because the switch will only forward the public IP to a host, each host plugged into the switch needs its own IP to communicate

[Belize]

Uh oh, sounds like someone is going through Networking and Database class...

[Cyanotical]

also, the SB6141 does not have a hidden router built in, it has a small DHCP server that will give you a 192.168.100.x address if your ISP is not assigning IPs by dhcp

this is only really so that you can reach the webgui for diagnostics, if you get a 192.168.100.x address from your 6141, keep releasing it until you get a public IP