I'm a bit confused, and was wondering if someone can set me straight. Under a Local Area Network (LAN), what is the communication protocol for the data packets under i.e. ethernet CAT5 cable that seems to be the common practice. Is it TCP/IP? But if the internet isn't utilized (no modem but just router) is it still TCP/IP?

Thank you in advance!

  • This feels like a test question? – xeon Mar 2 '12 at 20:59
  • ahaha.. just trying to fill the holes in my knowledge of networking.. – O_O Mar 2 '12 at 21:01

Usually, along with UDP and ICMP which are probably the most common, but many other protocols exist and are used too.

Many protocols that are considered to be legacy are still actively used today on some networks. I frequently work with legacy systems which use protocols such as LAT, DECNet, IPX.

This Wiki page explains this broad topic in a fair amount of detail, and should help you gain the knowledge you are seeking.

  • 1
    +1 with the caveat that UDP and ICMP are a part of the TCP/IP protocol suite. – joeqwerty Mar 2 '12 at 21:03
  • @joeqwerty - Well, wouldn't that be UDP/IP and ICMP/IP then? – EEAA Mar 2 '12 at 21:08
  • Well... I mean they're protocols in the suite of protocols that is the TCP/IP protocol suite. – joeqwerty Mar 2 '12 at 21:25

In general, yes, it is TCP/IP. Even if there is no connection to the internet, TCP/IP is still in use for the internal network.


For the most part, yes even private networks use TCP/IP at this point. It has pretty much taken over from all the old protocols - IPX, AppletTalk, DECNet, NetBEUI and so forth.

You may still see some of these old protocols around but their use has deminished significantly. These days the only place you won't see TCP/IP is for highly specialized uses such as InfiniBand or Fiber Channel networks.


Yes. The Internet is just a big network with public IP addresses versus a small network with private IP address mostly found behind a router.


In the transport Layer of TCP/IP the vast majority of actual data packets that will be traversing any LAN will be:

TCP (Transmission Control Protocol), UDP (User Datagram Protocol), ICMP (Internet Control Message Protocol)

These are differing methods of encapsulating data before it is passed onto the network layer.

There's other layers at play, all working together but I believe this is enough to straighten you out for now.


You mention LAN, ethernet, TCP/IP and internet, so it's not 100% clear which layer of the stack you're talking about. There are multiple layers that each have their own set of defined protocols.

Take a look at the wikipedia page for the 7-layer OSI model: http://en.wikipedia.org/wiki/OSI_model

At the physical layer, you may be talking rj45. At the datalink layer, if you're talking about wired LAN, then you're probably taking about IEEE 802.3 or 802.11 for wireless (there are others). At the network layer, you could be talking about IP, IPsec, ICMP, IGMP, OSPF, etc. At the transport layer, TCP and UDP are the common ones.

You didn't mention bits of the stack about that, specifically, but there are, by the OSI model, another three layers.

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