Network Types vs Network Topologies

There are many different computer network types out there.  Since you are on this website, you have to be a part of the biggest computer network in the world: the World Wide Web.  The WWW is an extreme computer network.  It consists of millions of devices from all over the world which includes servers, PC clients, phones, game consoles, routers, switches, modems…you get the idea.  If a device provides services for the Internet (such as a web server) or retrieves services off the Internet (such as your computer,) that device is part of the World Wide Web.

However, there are different network types which have a scope at a much smaller level.  When most people think about a computer network, they think about a computer network inside a building.  If you own a router (wired or wireless,) you have your own network.  This type of network is called a LAN (Local Area Network.) A LAN is a computer network that connects two or more devices together within a constrained area, usually a building.  If you have two or more computers that you share files with at home, these computers are on a LAN.  If the company you work for have computers that can share files or network resources (such as Internet or printers,) they are on their LAN.

Usually, local area networks need to be able to communicate with each other.  There are many reasons for this.  For example, you are on a computer which is on its own LAN.  In order to see this page, your computer has to go out of the bounds of the LAN it is on so that it can access the web server of this website, which is on its own LAN.  Hence, your computer needs to access a WAN (Wide Area Network.) A WAN is used to connect LANs together, so that they can communicate with each other.

Sometimes, there might be a company that does not want some of its computer resources accessible over a WAN.  However, this company might have a few sites within the same town.  So, the company might be using a MAN (Metropolitan Area Network.) However, there might be a company that have sites all over the globe.  A great example would be a fast food chain.  The only practical thing to use would be the Internet.  So, a company like this might connect all the sites together using secured ‘tunnels,’ so that even though these sites are not connected to the same LAN, they can act like they are through these tunnels.  This is called a VPN (Virtual Private Network.) Last but not least, another network type worth mentioning is a SAN (Storage Area Network.) The purpose of a SAN is for an enterprise network which has a network dedicated just for data backup.  Since most home users do not have a SAN, I will kind of skip over this type .  Most home users do not have their own MAN, either.  So, I will not get into MANs as well.  I might get into VPNs in more detail in later posts.

WAN, MAN, and LAN are different network types.  Bus, star and ring are different topologies.  Topology is the method (or layout) used to connect computers together on the same network.  While layer 1 describes the physical characteristics of the cables themselves, a topology describes the pattern of how the cables get connected together.  Do the cables meet at a central point?  Does every computer get directly connected to every other computer?  Usually a specific layer 2 protocol and medium is used for a specific topology, however this is not a requirement to define a topology.  Depending on the circumstances, a layer 2 protocol might be used for more than one topology.  Medium type might be used in multiple topologies as well.

Network topologies are, in general, part of the layer 2 of the OSI model.  The reason for this is that different topologies use different layer 2 protocols.  Also, in order to cross over one topology to another, you need at least a layer 2 device in order to accomplish this.  A device used to connect multiple topologies together (usually two) within the same network is called a bridge.  Bridges have different purposes as well and will be explained in later posts.  For the purpose of this post, however, a bridge is used if you want to send data from one topology to the next on the same network.

Notice that I mentioned twice that a bridge connects topologies together on the same network?  I want to make that clear because a different device is used to connect different networks together.  A bridge is a layer 2 device because it makes its decisions based on layer 2, via the MAC address.  Lets look at this scenario:

Computer 1 and computer 2 are on topology A.  Computer 3 and computer 4 are on topology B.  Topology A and topology B are connect via a bridge.  The bridge learns the MAC addresses of computer 1, computer 2, computer 3 and computer 4.  If computer 1 sends data to computer 2, it is sent to computer 2 and the bridge.  Since the bridge knows that computer 2 is on the same topology as computer 1, the bridge does not forward the data onto the other topology.  However, if computer 1 sends data to computer 3, the bridge will forward the data onto topology B.

Makes sense at all?  A LAN is generally a network inside one building (such as your house.)  A WAN is used to connect multiple LANs together (such as the Internet.)  A LAN can use multiple topologies.  In order to connect the multiple topologies together, a bridge must be used.  Even though a LAN may consist of multiple topologies, a topology does not cross multiple networks.  For example, a LAN connected to a WAN does not use the same topology.  One topology is used for the LAN and another topology is used for the WAN.  Why would a LAN have different topologies?  It can be a way to converge an older network with a newer network.  Usually it is used for newer computers using a star topology to communicate with old legacy computers using a ring topology (or to a proprietary network.)

Resources:
http://www.fused-networks.com/information/100488/convergenceglossary/
http://www.pcmag.com/encyclopedia_term/0,2542,t%3DVPN&i%3D54123,00.asp
http://www.pcmag.com/encyclopedia_term/0,2542,t=SAN&i=50794,00.asp
http://www.its.bldrdoc.gov/fs-1037/dir-024/_3535.htm