understanding LAN/WAN/Site

1- in LAN we can have one or more subnets, and the subnets can be in one building, or many different building spread over a geographic area. Correct?

If so,my question would be  what 's the difference between LAN and WAN.

2- Site topology is based on subnets. Correct?
My questions is what determine that  we need one  site[intrasite] or many [ intersite]?
How physically ,intrasite/intersite, are  setup? an example would help.


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The very definition of LAN (Local Area Network) and WAN (Wide Area Network), and even the expanded MAN (Metropolitan Area Network) have become blurred with the advancement of fiber optic communications and fast switching.
The topology is purely layer 1 and layer 2 determined.

Basically, we can classify a LAN as a single Ethernet network with all switches/hubs interconnected. That interconnection can be over long-haul fiber cable (layer 1), but it all appears to be the same local LAN no matter where you are (layer 2). It can be all one big broadcast domain, and all controlled and managed by your company, typically to include the fiber optic cabling.

You can add on layer 3, creating VLAN's at layer 2 and routing between vlans all within the same LAN no matter how big that LAN really is. VLAN's simply define broadcast domains and are typically subnet boundaries at the same time. This will allow you to create your own Campus LAN or even a Metro LAN depending on the fiber infrastructure.

Typically, a WAN is created by using some other method of transmission of data because you can't get direct fiber links. Not many companies can afford to bury their own fiber optic cabling between London and New York. Examples include T1, T3, ATM OC-3, broadband, dsl, VPN, MPLS, etc. - all technologies used by the long-haul specialists like telephone companies and Internet backbone providers. Now you need some device that converts Local Ethernet into some other long-range encapsulation method to transmit across the (semi-public) long-distance trasmission line. This is typically where a router comes in. Any time you have a router that connects two sites, you typically have full subnet boundaries (but it's not required) and, by definition, broadcast boundaries.

So, it's really the technology that connects the sites together that defines a LAN or WAN.
In a LAN, you own and control all connecting pieces.
In a WAN, you (buy, rent, lease, purchase) data connectivity services from someone else to connect your sites together.

An example might be a large business campus, or even a University campus. All the buildings within the campus are interconnected using fiber optic cables, and the business owns, maintains and operates everything together. But, this same company has several remote small offices (sales offices, Dr. Offices, etc) that need to access data that is on the Campus LAN. This is where we buy a T1 (as an example) from the local telco, put a router on each end and provide that remote office with access to the LAN-based services/servers. Let's say we have 100+ remote offices to support. Each one buys some transmission media (T1, fractional T1, DSL, etc) and a router to connect bak. Now we have a WAN. Doesn't matter if just one remote site or 100's.. It's the fact that we don't own or control what's in the middle..

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1) WAN means Wide Area Network. LAN means *local* area network.

Generally, if your network spans more than one site I would suggest it's a WAN. But the distinction is only important to the network manager, though it will help users to know that sources at other sites may be slower to access than local ones.

2) Subnets are a way of dividing up your IP space into separate IP networks. These can be used at different sites, though there may be reasons to use more than one IP network at one site.

However, trying to "bridge" a single IP network across multiple sites may not be advisable, it's better to use a router to do this and minimise the "stray" traffic on teh WAN link as it won't be as fast as your LAN.

So you may have something like

London office Main Building - Site 1
London office Second Building - Site 2

Site 1 links to Site 2 via 10Mb SHDS link, router on each end.

Paris office is site 3

Site 3 is linked to Site 1 using a VPN, via Site1's 2Mb leased line and Site 3's 1Mb/256Kb ADSL service.

I would be using a private IP range for each site - so say in Site1, in Site2 and in Site 3. Sites 1 and 3 have internet links and will have one or more IP addresses from the respective ISPs.

ChuckbuchanAuthor Commented:
thanks a lot for your explanation about LAN and WAN.

I might need some more explanations about the second question

2- Site topology is based on subnets. Correct?
My questions is what determine that  we need one  site[intrasite] or many [ intersite]?
How physically ,intrasite/intersite, are  setup? an example would help.

thanks again
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>2- Site topology is based on subnets. Correct?
IMHO, site topology is based purely on physical constraints.
When talking topolody, I think physical - star, bus, etc..
Has nothing whatsoever to do with subnets.

I gave you an example of an intrasite (campus) and intersite (remote offices talking back to the campus)
Physically, an intrasite (campus) is connected together using any standard Ethernet media, the distances being limited by the available media (fiber vs no connectivity)
Intersite (remotes to campus) connectivity typically requires purchase of some data transmission media from the local telphone company.
ChuckbuchanAuthor Commented:
what I understand from your explanation is:

if the connectivity media( what is in the middle) is controlled by us , we considere this as an intrasite.

if the connectivity media( what is in the middle) is controlled by phone company, or telecom, we considere that as intersites.

for the subnets on which the site is based on, can they be from the same class? example:

 a california site has a subnet that starts from and have 50 users,  and
new york site has a subnet that starts from to, and the 02 sites are linked.  
Is this possible?


>a california site has a subnet that starts from and have 50 users,  and
new york site has a subnet that starts from to, and the 02 sites are linked.  
Is this possible?
Yes and no.
Yes, they can be bridged together so that they are all on the same IP subnet and appear as though they are on the same LAN.
Does this further blur the distinctions between intra-site and inter-site? Absolutely, but we're talking apples and oranges.
Apples being practical examples of LAN and WAN, and 'traditional' way of thinking to classify them
Oranges being examples of Intra-site vs Inter-site connectivity..
Think about the terms intrastate and interstate commerce.
Everything that happens within the sate borders is intrastate
Anything that crosses the state borders, is interstate
Now, where do you define the borders for a LAN (inter-site) and a remote site over a WAN if you can bridge them together to make them appear as one. Still inter-site, or intra-site? How about if you have 3 remote sites all connected by some 3d party telco, but bridge the links and they all appear as one big happy LAN, all on the same subnet? Is this inter-site communications or intra-site communication? Here we've used technology to define a "site" as one virtual site that encompases 3 physical sites.
Take that concept a step further. Suppose they all have different IP subnets, and they are simply routed, not bridged, yet they are all a part of one Windows Active Directory tree and can see/share everything as if they are all in the same campus lan. Does this change the definition of inter-site and intra-site? I believe it does to some degree because there must be put into place some other mechanisms to make all the intra-site traffic appear as one inter-site.

No, the two sites can't use those ip address boundaries because they are not true subnet boundaries.
CA can have (62 hosts)
LONDON can have (62 hosts)
NY can have (127 hosts)

Now they are truely separate IP subnets and can talk to each other without creating a bridge, yet they are all part of a single subnet of a single Class C network.
But, this leads us into a discussion of pomegranites after the apples and the oranges..

... punt....

ChuckbuchanAuthor Commented:
I know you have taken long time to explain this.
let me make sure we are in the same page : creating one site or multiple sites depends on:

*the reliable  connection speed: if we have T3 connecting CA- NW-FL we can have them all in one site regardless if they are in the same or different subnets, and we can use either RPC/IP(synchronuous) or SMTP (asynchronuous).

 if we have unreliable connection then we need to create different sites for replication and bridge them.

WAN = media that you lease
LAN = media that you own

That being said, if you can own it cost effectively, that would be the best choice and would provide faster transmission but that is almost always with fiber or copper (limited distances, ie. think campus).

If you HAVE to lease it from a teclo or other service provider (SP), then it's a WAN.

Now, your last comment makes it sounds like you are talking sites a-la-Microsoft Active Directory (AD)...

For that, see this link... it has detailed design guidelines for sites and replication of domain controllers.  In particular, see "Designing and deploying Directory and Security services" and within that, "Designing the site topology".

This is way more info than we'd be able to share with you here.  I hope you find that useful.
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