How to move a computer onto a switch and keep the IP address?
April 27, 2013 3:57 AM   Subscribe

I have a Mac Pro set to DHCP manual assign and a certain IP address that cannot change. I also have a printer plugged into a certain network jack; IP address less important. I am moving the physical location of the Mac Pro near this printer and the printer's network jack is the only one available. I'll have to plug both of these machines into the same network jack via switch/router. What switch/router is the right one to buy in 2013 (bonus if I can find it at Best Buy) and what the right way to set this up so the Mac Pro is still reachable at the same IP address on the network?
posted by michaelh to Computers & Internet (12 answers total)
You can buy pretty much any unmanaged switch you can find. You don't need a router because the printer and computer will very likely already both be plugged into the same network; ask your network admin or IT guy if you think this might be an issue. Here's one that is complete overkill — you'll never saturate a gigabit switch and the network jacks it provides will only be as fast as the port you're plugging it into, but it will work like a champ. Best Buy is ridiculously overpriced; you'll get a much better deal from Amazon or Newegg for this device. Note that you'll need an extra network cable to plug the switch into the wall jack.
posted by Blazecock Pileon at 4:16 AM on April 27, 2013 [1 favorite]

Buy any cheap gigabit ethernet switch. Switches don't affect IP addresses. Everything should work fine assuming you're not on some locked down, port managed network (if there's no dedicated network staff managing things, you'll be fine).
posted by ryanrs at 4:21 AM on April 27, 2013

This is the cheapest one ($25) at - you'll have to click the "Check stores" to see if it's available near you if you absolutely must buy it from a brick-and-mortar.

For just a couple more pennies, Monoprice has a faster switch, and they have the ridiculously cheap network cables that you'll need, as the Blazecock mentioned above.
posted by panmunjom at 4:41 AM on April 27, 2013

Getting a 100Mb switch like that one at Best Buy is probably not a good move. Get a gigabit switch.
posted by ryanrs at 5:18 AM on April 27, 2013

$20, same as in town
posted by flabdablet at 5:57 AM on April 27, 2013

Assuming your network is simple, the switches and physical ports have nothing to do with the IP address assignments. The IP address is assigned by your DHCP server, which on a small home network is probably your router. Most DHCP servers will try to give the same computer the same IP address when it turns on; it identifies the computer by the MAC address, a unique identifier of the ethernet or wireless adapter.

So the question turns entirely on your DHCP server. If by any chance you're using the Tomato firmware or DD-WRT or some other good third party firmware, they have a screen where you can statically assign IP addresses to specific machines to ensure they always get the same address. That's a better solution than hoping the machine doesn't lose its address. Another option is to configure your Mac with its own IP address, not to use DHCP. Be sure to pick an address that's not in the range the DHCP server will be serving (which means you need to change it once, now.)
posted by Nelson at 6:46 AM on April 27, 2013

Oh, and if for some reason you don't have a router at all on your network, buy an ASUS RT-N16 and put EasyTomato on it. But I imagine your network already has a router (and DHCP server); you don't need a second one if so.
posted by Nelson at 6:47 AM on April 27, 2013

The way you can think about your home network is as one 'wire', most of the time. This is a holdover from when each Ethernet segment actually was one wire, strung along a line of computer stations; they had a T joint for each station, with a single strand of coax running across each hop. Even though we've gone to better wiring systems (first hubs, and then switches) which actually physically look more like a star pattern, the layout has stayed the same: typically everything in a home network will all be on 'one wire'. Then, you'll have a router; routers are the demarc points between network wires. Most home networks have an internal wire, a router, and then an external wire that connects to the Internet. Many well-connected homes may have a dozen or more devices on their internal wire.

From your terse description, it sounds like you've got a central router, with some switch ports, and you're running clients to your one switch through network wall jacks. All your devices are on your one network wire, and everything has a dedicated port, so everything has just been working. But, now, you want to multiply one wall jack into two. So, to do this transparently, you want to attach a switch to that port, not another router. Routers are demarc points to make new network wires. Switches just extend a logical wire that's already there. That's what you want, so use a switch.

(Most small routers also have onboard switches, so you can use them in the same way, but A) they're more expensive, so you're usually wasting money, and B) it's easy to wire them in the wrong way, so that you're breaking your one network wire into two or more, and C) if you don't forget to turn off all the services on the new router, it may interfere with the DHCP and DNS service being provided by the old one. If you've already bought a router, you can almost certainly make it work, but you really want just a switch. It's much easier.)

If you get a gigabit-class switch, you should be able to just plug it into the wall port where the printer presently plugs in. If the port on the switch lights up, you are good to go. Just plug the printer and the Mac Pro into other switch ports, and everything should just work. You've extended your wire a little bit in that one location; you can add more devices to that switch, if you wish.

You shouldn't have to think about IPs at all. The IP layer is on top of your network wire. Since you used a switch, and just extended the wire a tiny bit, everything that worked before should keep working exactly the same way. The IP layer should have no idea that anything even changed.

If the switch port doesn't light up, or if you get a Fast Ethernet switch (10/100), you may need a 'crossover' cable as well. Fast Ethernet has two plug types, 'router' and 'client'. It's designed to always work when connecting a router port to a client port. If you're connecting two clients, or two routers (or two switches -- they use router-type ports), then you need a crossover cable to make it work.

Gigabit has a feature called 'Auto MDIX', which is supposed to figure this out, and just work, so you can use any kind of cable to plug between any types of port. But if that's not working, or if you don't have gigabit, you might need a crossover.

Monoprice is a very good source for both good cheap switches and excellent cheap network cables. I haven't used their switches, though I've heard they're good, but I've used many of their cables, and like them a lot.

If you think you might need to expand much further in that location, you might want an 8-port switch. With a four-porter, one port will go to the wall, one to the Mac Pro, and one to the printer, so you will have one port free for further expansion. If you use an 8-porter, you will have five ports free.
posted by Malor at 10:56 AM on April 27, 2013 [1 favorite]

Oh, and Blazecock caught a detail I missed... if this is actually a work network, then you probably shouldn't be plugging anything in without IT help. If you absolutely insist on doing so anyway, then everything I said will probably still apply; buy a gigabit, unmanaged switch (that's important, as managed switches, or routers, can be security holes), and plug it into the existing port. If you get a light, you're good to go: plug the Mac Pro and the printer into the next available switch ports.

In a commercial setting, it's somewhat more likely that the printer port and the Mac Pro are not on the same logical wire. If that's the case, the network may 'lose' the Mac Pro at the new physical location, not knowing where to find it anymore. It's common for a whole floor, sometimes even the whole building, to be on the same logical wire, but there are a number of performance or security reasons why it might not be.

That's why you'd want help from IT.
posted by Malor at 11:17 AM on April 27, 2013

Check your current connection on your Mac to determine what speed it is connecting with, 10/100/1000 Mb/s. There are plenty of places still wired with Cat3 wiring that would only reliably do 10/100. Some 10/100 switches have a special 'uplink' port that removes the need for a crossover cable (uplink port goes to wall jack with regular cable, computer/printer go on the non-uplink ports with regular cable). And yeah, at my workplace we have printers on a separate network from computers and a policy of 'only one machine per port' that would get your port disabled for doing this, so check with your IT people if you have them.
posted by zengargoyle at 12:44 PM on April 27, 2013

Thanks, everyone; good recommendations. Thanks to Malor especially for allaying my fears about how switches might work with the IP address. I'll see how it goes on Monday morning.
posted by michaelh at 5:51 PM on April 27, 2013

The switch is in place. The computer is still accessible by the same IP address. Everyone can print without issue. Thanks, everyone!
posted by michaelh at 3:25 PM on April 29, 2013

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