What is the difference between a modem/router/switch/WAP ?

To understand the difference between a modem and a router, you need to understand several different, but related, devices.

The individual devices are

 (1) an ADSL modem; (2) a router; (3) a switch; and (4) a wireless access point (WAP).

It becomes confusing because there are combination devices which can combine both an ADSL modem and a router in the one case as well as devices which can combine an ADSL modem, a router and a switch all in the one case!

What is an ADSL modem

An ADSL modem is a device that transmits and receives data over the plain old telephone system's (POTS) analogue lines. It allows your PC to connect to your ISP using the Internet Protocol. The modem acts as a "bridge" between your PC and your ISP. The dynamic or static public IP address which your modem obtains from your ISP is passed on to your PC. Your ADSL modem forwards packets based on hardware level MAC addresses between your ISP's router (and the Internet) and  PC.

What is a router?

A router is a device which acts as an interface between two networks. It forwards packets based on network level addresses (Internet Protocol addresses in this case) between your ISP's router and either your LAN or your single PC. A router learns more about the networks to which it is connected and can be more selective about the packets it passes on. A router rejects packets unless they match predefined attributes (eg specific protocols or destination network addresses). A router can also select the best route for packets to take in large interconnected networks.

What is a switch?

An Ethernet/LAN switch creates a virtual network between two networked devices for the duration of the data transfer. This is unlike a hub which forwards all received data to all connected devices, even though the data may be destined for just one of those devices. A switch learns the association between the MAC addresses of connected devices and its switched ports. By sending data only to where it needs to go, a switch reduces the amount of data on the network, thereby increasing the overall performance of the connected devices. A switch also improves security, since data is not broadcast to every connected device, but only to the device for which it is destined, it cannot be monitored by other connected devices. For a more detailed look at how switches work, check out this link at the How Stuff Works web site.

What is a WAP?

A Wireless Access Point (WAP) is a 'base station' device that connects a wired Ethernet network to a wireless network (WLAN) comprising one or more wireless devices. While very small WLANs can function without a WAP in "ad hoc" or "peer-to-peer" mode, WAPs support "infrastructure" mode which bridges WLANs with a wired Ethernet network and also scales the network to support more clients.

There a few different types of WAP: (1) Bridges ? connect devices that all use the same kind of protocol

 (2) Hubs ? provide networking connections to a variety of clients and add features like roaming

 (3) Routers ? connect networks that use different protocols

 (4) Gateways – provide additional features such as NAT, DHCP servers and firewalls.

• Inbuilt PPPoX login clients – so that you do not have to setup separate PPPoX login client software on your PC and can maintain a connection even if you turn your PC off; and

• Network Address Translation (NAT) – but for a very limited number of PCs (eg only 2 PCs, whereas if you were using a router which does NAT you would be able to do it for up to 253 PCs).

Pluses and minuses of the combined approach

Pluses for the combined modem/router and modem/router/switch devices are that you only have to find a home for one plugpack, you do not lose half your desk space to three new boxes and that there are fewer cables for you to have to hide out of sight or otherwise manage.

Minuses for the combined approach is that if any one of the devices in the combined unit malfunctions, you will have to return the whole unit to the manufacturer or, if your unit is no longer under warranty, you will have to replace the whole unit.

 If one wants to replace one of the devices (eg upgrade your modem to ADSL2+, upgrade your WAP to 802.11g etc), you have to replace the whole combined unit which, again, will generally be more expensive. It may also be more difficult to troubleshoot an all-in-one device – with separate units you can physically remove one or more from the equation and isolte the problem more quickly.