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Most of explanations on internet contend by saying "lan" is over smaller distance, while "Wan" covers wider geography. To make question more clear ?

  1. If two computers are on the same "LAN" what is it that they share in common to be on that single LAN they belong to ? Is it that they share a common router/switch/subnet ? Technically what is that that unifies under a LAN ?

  2. If two computers are on the same "WAN" what is it that they share in common to be on that same WAN they belong to ? Is it that they share a common router/ subnet ? Technically what is that that unifies under a WAN ?

  3. Is single datacenter considered a Lan ?

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Wikipedia is your friend:

LAN: A local area network (LAN) is a computer network that user interconnects computers in a limited area such as a home, school, computer laboratory, or office building using network media.

In simple words, what unifies devices on one LAN: they are situated in the same area. Most commonly, one LAN is behind one router (not talking about backup routers and redundancy routers).

WAN: A wide area network (WAN) is a network that covers a broad area (i.e., any telecommunications network that links across metropolitan, regional, or national boundaries) using private or public network transports.

Again, in simple words: a WAN connection is connecting LAN's over a bigger area.

Example: LAN + internet/network + LAN = WAN (when talking about bigger area's)

  • If I understand you correctly, if two routers were serially connected in the same room, the link would not be a WAN. It would only be a WAN until the routers are in a seperate geographic location e.g. different cities or even countries. However, in a question I asked a while ago networkengineering.stackexchange.com/questions/17270/how-do-i-count-lans-and-wans-given-a-network-topology , the accepted answer states that any serial connection should be treated as a WAN (so that would include two serially connected routers in the same room). Did he make a mistake? – user1534664 Jan 19 '16 at 1:04
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There is no 'technical' difference. Traditionally they used quite different networking technologies although that is becominging less and less the case.

The vague distinctions are that a LAN is in one local area, with lower latency, always connected, generally controlled by a single body, a WAN is spread over a distance and expected to have higher latency connections which need to be established (eg. vpn tunnels, dialup, etc.)

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In the old days a LAN was a network built for local communication within a relatively small area (such as a building or maybe a campus), it used technologies like Ethernet, Token ring and so-on. Since local links could be fast and cheap simplicity was a priority over link efficiency.

A WAN was a network built to connect systems over a large area, generally by interconnecting a number of LANs using point to point circuits rented from a telco. Scalability and efficient usage of links were priorities over simplicitly.

But the lines have got much blurrier over the years. The move from CSMA/CD to full duplex point to point links removed the distance restrictions from Ethernet. WDM allows links to be electrically independent while travelling over the same fiber. Various protocols have been introduced that encapsulate ethernet packets to carry them over a wider network. It is now practical to build networks that look like a single ethernet network despite spanning large geographical areas.

In the other direction the much larger number of users in a building/campus and increasing security concerns has meant that a single flat network on a building/campus is less practical than it used to be. Technologies like VLAN have been introduced to split them up.

So while geography still has an impact on network design the distinction between "LAN" and "WAN" is nowhere near as hard-edged as it used to be.

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Just a comment on distance: although, of course, with technological progress bandwidth is increasing at distance, there are some insurmountable fundamentals on lag which are not going to be overcome.

It is 17,000 km from London to Sydney by great circle distance, which is 0.057 light seconds. This means that the absolute minimum ping time is 114 ms. If the path isn't straight, there are signal propogation delays or there are packet forwarding delays it will be longer. If you have a lag-sensitive application, such as some database systems, it won't be possible to overcome the problems, no matter how much money might be available.

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LAN:

Single LAN for Eg: group of device like pc,printer.. connected to switch i.e Single LAN.

Ethernet is by far the most commonly used LAN technology

if u want to create multiple lan in single switch ,that is the concept of VLAN.

WAN:

The Internet can be considered a WAN as well, and is used by businesses, governments, organizations, and individuals for almost any purpose imaginable.

WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations.

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    A WAN doesn't have to belong to an ISP. Even the connection between 2 LAN's (to make a WAN) doesn't have to belong to an ISP. You can also have multiple broadcast domains in a LAN via VLAN's. – Bulki Feb 17 '14 at 7:12
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Original Answer

As a rule of thumb, a "LAN" is a broadcast domain. (It may consist of more than one.) Then again, as a rule of thumb, a rule of thumb applies 3/8ths of the time, so this doesn't really give much to go on.

This was understandably rather poorly received, so I've expanded below.

Updated Answer

I've never really liked the essentially recursive answer of "a LAN is a network of devices in the same area." Granted, it's a rather subjective term, so here are a few other options to illustrate what one may be. Note they may not apply in all situations, there may be exceptions... or they may ALL apply to your situation:

Layer 2 Network If devices are in the same Layer 2 Network, they can be described as being on a LAN.

Broadcast Domain Devices in a broadcast domain are in a LAN (or multiple LANs). This normally applies to VLANs (Virtual LANs), which can be used to segment a LAN... or can be used to link physically distant devices into essentially the same LAN. Yes, this stretches the definition of "local!"

Non-Routable IP Address A set of devices on the same non-routable IP address subnet who can talk to each other (without NAT or other workarounds) are most likely on the same LAN as well. What are non-routable IP addresses? Wikipedia explains it well enough: http://en.wikipedia.org/wiki/IP_address#IPv4_private_addresses Essentially, these ranges are set aside for "local" use and are not to be forwarded out to the internet.

Sidebar These non-routable ranges appear to have been rather arbitrarily chosen--eg, there's nothing special in electricity that makes 10.0.0.0/8 range not route, but someone realized some ranges should be set aside and made a judgement call about which ones. I could be wrong (which isn't rare by a long shot), but Jodies.de lets us see the binary of these ranges ( http://jodies.de/ipcalc?host=10.0.0.0&mask1=8&mask2= ). I don't see anything terribly remarkable about the addresses. They just seem arbitrary. See for yourself:

Address: 10.0.0.0 00001010 .00000000.00000000.00000000 Netmask: 255.0.0.0 = 8 11111111 .00000000.00000000.00000000

Address: 172.16.0.0 10101100.0001 0000.00000000.00000000 Netmask: 255.240.0.0 = 12 11111111.1111 0000.00000000.00000000

Address: 192.168.0.0 11000000.10101000 .00000000.00000000 Netmask: 255.255.0.0 = 16 11111111.11111111 .00000000.00000000

Enough digression!

VPN Tunnels Additionally, a VPN tunnel (be it just from your PC to your company or a site-to-site/LAN-to-LAN tunnel) lets a remote machine behave as though it were on the LAN. If it walks like a duck, and it talks like a duck, and it tastes like a duck...

Conclusion "LAN" and "WAN" are rather ambiguously defined. They are hardly the only delineations, either--I've been trained on a model that went from LAN to optional CAN (Campus Area Network) and/or to MAN (Metropolitan Area Network) to WAN. You may define your LAN as everything behind your home internet modem, but your ISP could consider every subscriber on your block to be in a LAN... and they could both be right simultaneously! The terms can subjective to a degree, but these guidelines (and all the others posted) should help you determine if something is a member of a LAN, a WAN, or both.

  • This is closest to the answer I would provide, which in summary is that the definition of "LAN" and "WAN" like many words is highly dependent on the context in which it is used and the parties involved. – YLearn Sep 18 '15 at 16:54
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There is no technical definition sadly, but i found this useful and rarely told:

The focus in a LAN is that it interconnects hosts while in a WAN it interconnects connecting devices.

Being host previously defined as an end system. Examples: desktop, laptop, printer, cellular phone, etc.
And connecting device a.k.a. Networking Hardware. Example; hubs, switches, routers, bridges, etc.

A LAN is normally privately owned by the organization that uses it.
A WAN is normally created and run by communication companies and leased by an organization that uses it.

Source: Data Communications and Networking book from Behrouz A. Forouzan.

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There are three types of networks:

  1. Enterprise Networks - Enterprise networks basically are the most basic and common networks that exist, these networks have a simple topology, mainly consist of network switches, and router facing the internet connected to the ISP. The switches may vary from layer 2 or layer 3 switches and VLAN configured.

  2. Data Centers - Data Center networks are the second most complex networks, most big internet startups have their own data centers, i.e Google, Facebook. Data Centers have high end switches and routers for optimal speed and performance. In addition technologies such as VMWARE data center virtualization helps in maximizing performance and limiting the physical devices that runs in this network.

  3. ISP Networks - ISP networks are the most complex networks that exist, these networks operates in the WAN or the "internet". It is important to note that the largest WAN is the "internet" and it is a network of networks that is decentralized.

In conclusion, think of a LAN as your home network, a WAN as your ISP.

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