What's the difference between a network switch and a router?

The basic answer is: you must have a router, a switch is only used when required. Routers manage the network, they've got an OS (just like your PC does) and are necessary. Switches are used to add more connection points to a network. If you can wire your office just the number of ports on router, there shouldn't be any reason to use a switch.

A switch is a layer 2 device while a router is a layer 3 device. What this means is that a switch controls the flow of ethernet packets (layer 2) while a router controls and directs TCP/IP (Internet) packets. A router is used to connect your office to your ISP. A switch gives you the ability to segment your internal network. As Mr. Baker pointed out most small office routers have several switch ports on them. If not, you can just attach a hub which is a layer 2 device that extends the number of ports on your network and is very inexpensive (less than $100).

If , in your leased office , your are planning to own a three PCs/Mac and have 3 Voip systems then you will need to buy a switch and a router since many routers do not come with enough connections;

To set the picture for your and to describe where things sit in that picture here is a fairly wide introduction to these terms.

Your going to have a network , which means your planning to have one or many devices which will be connected to each other ( and possibly the internet ) by way of physical wiring referred to as network cabling.

That network cabling is going to come out of every device and ( if your office is correctly cabled/wired up ) connect to a socket in the wall. That socket in turn is just a longer network cable which end up in one room in the office where all the network cables are numbered to represent the rooms/panels in the office.

If all you had were a few devices all going to see each other and no concern for the internet then you will get by with an 'Unmaged Switch' sometimes sometimes incorrectly referred to as a Hub this will provide 5 or 8 or 12 or 16 or 24 or 48 network cabling sockets. Note a unmanged Switch is preferable to a Hub and unmanaged switches have become incredibly cheap to the point that Im not sure I can actually buy a hub any more. [ essentially it sounds like you can have an umanaged switch ]

If you plan to connect the devices to each other so that they can see only the devices they are supposed to see and not each other then you could buy several unmanaged switches, one for each network of devices your connecting. [ note this is not what you described as needing but it sets the tone for Managed switches ]

Alternatively you could buy 1 Managed Switch which allows you to create several networks ( on Managed Switches these are usually referred to as VLANs). Now you have less space taken up in the networking room and one device to manage all the networks in your office. Managed Switched are generally twice to three times more expensive than their unmanaged counterparts. [ your not planning to do this but I need to set the picture of why its useful ]

Since your looking at Vo-ip its important to note that you might need to have a UnManaged Switch that provides Power over Ethernet for the Handsets On Many managed switches no all network ports on the switch carry the power so check the Features to see how many will be powered.

Now we have discussed how to connect your devices into one or many networks your next goal is to possibly connect them to the internet or to another network at another office or location.

A Router provides a networking device to carry your network information out of one network to another. However in the more common offfice based terminology people are talking about Routers in terms of going from the Office to the Internet. You will need a Router to connect your office to the Internet and if all your are planning is one network where all the devices can see each other then many off the shelf Routers do the job just fine (* see firewalling and IP addresses ) . [ this is essentially what you have described ]

If your planning a more complex collection of networks allowing different networks in the office to access the internet then there is some additional planning required and its work purchasing a Cisco Engineers time to configure up a more expensive router.

TO RECAP

A Router:
Will Connect your office to the Internet ( as the most basic office description goes ) it will have one to either network ports for connecting office devices to the router in the same way that an unmanaged switch will. Generally you connect the router to the switch to connect the network to the internet.

A Switch
There are two flavours of Switched : Managed and UnManaged. Managed Switched provide more complex network setups. Similarly buying one or more unmanaged swtiches can get similar results. Some people incorrectly refer to these as hubs since if you remove the switching part of the device it is essentially a hub.

Firewalls
Many off the shelf routers such as netgears and zyxel come with Firewalls that are more than sufficient for office based networking. In many cases though you may need to allow access for mail inbound which requires further thought regarding your network planning. your voip phones will certainly need to allow inbound internet connectivity.

IP Addresses
If your planning more than one network in your office then its important to ensure that each network is assigned a separate IP Address range. Every device will be looking for its own IP Address, the IP Address of the default gateway ( usually the router ) and the IP Address of one or more nameservers.

Your requirements sound like :

You need a network DSL router :
You dont mention if the leased offices provide internet access , many will and as such you just require a swtich. If they dont then for office sizes of upto 50 seats the netgear range of dsl routers are perfect for routing, firewalling and network IP address concerns.

You need an Umanaged Swtich :
Again check to see if you need an power over ethernet switch. Netgear again do a perfectly acceptable range of switches for SOHO that are very cheap

A router is only required if you need to connect your LAN to the internet for example.

If you only want to run a LAN without the internet, only a switch is required to connect them together.

As mentioned above, if you have lots of computers (more than will plug directly into a small router) then you need a switch as well, in order to connect them all together.

You will need to know about IP addresses. A typical LAN IP address is of the form 192.168.0.x. To talk to other computers on the network they will need the same beginning part (the NETWORK ADDRESS), and a unique HOST number ('x' in the example).

The subnet will need to be the same for all systems on the LAN. A 192.168.0.x IP address uses the subnet address 255.255.255.0.

Hope that helps to get you started. :)

In the early days of routers and switches the answer was that routers kept local traffic on the LAN (Local area Network) and let packets destined to addresses outside of the LAN out to the WAN. The WAN could be a web site or a dedicated connection to antoher site.

Now-a-days that definition is becoming blurred as LANs become more complex and switches become smarter. Again in the old days, switches just repeated the packets on all ports. Now, with the use of VLANs and other control features switches are taking on more and more of router like chores on the LAN