I wrote this for another forum - thought maybe someone would find it helpful.

Before I get flamed ... ... this is very BASIC (I know there are exceptions and over simplifications)!

From the beginning ...

For two or more computers to talk to each other they must be connected via a network. It is possible to have a very small network of only 2 machines.

When you have a small network - usually in one building, this is typically referred to as a Local Area Network (LAN).

Most "networking" on a PC is done with Network Interface Cards (NICs). These cards have the responsibility of changing the data into electrical signals which can be transmitted across a wire (cable) to another NIC on the other end of the cable.

It is possible to "network" in other ways, such as USB or Serial to Serial, but I'm not going to get into that. I will be restricting this to what is called Ethernet.

Ethernet is what is called a CSMA/CD protocol. This means, Carrier Sense Multiple Access with Collision Detection. What this means is that ethernet will listen to the wire. If it doesn't "hear" any traffic, it will talk. If it senses that it talked at the same time as another device, a collision has occured, and both devices will "roll a dice" and depending on the results, transmit again according to their "die roll".

In order for computers to talk to each other, they must share a common language. This common language is called a Protocol. Common protocols you might see re NetBEUI, IPX/SPX, and TCP/IP. NetBEUI is Microsoft's Protocol which is very small, fast, and easily configured (as in not configurable) - however it is not "routeable" which I will explain below. IPX/SPX is Novell's equivalent to TCP/IP. TCP/IP is a standard protocol used by many many systems - notably Unix - so it has become the "de facto" standard and is used for communication on the largest network ... the Internet.

So, you have 2 machines and 2 NICs and some protocol. What if you want 3 machines? You need some network equipment.

For Ethernet, this used to be a Hub. Hubs are still available, however as technology marches on, the cost of Switches have been dropping - so even if says it's a Hub, odds are it's a switch.

Hubs will receive traffic on one port and send it to all ports relying on the destination to respond.

Switch will receive traffic on a port and make a note of the source. As it does this, it will develop a table of what address lives on what port. Thereafter, it will consult its table of address and "switch" the traffic over to the appropriate port. Because of this technology, switches are able to reduce collisions on the network - a collision is when two machines attempt to transmit at the exact same time. Switches also typically allow full duplex meaning that the switch and the PC can transmit and receive at the same time. These reasons are the basic reasons why switches are better than hubs.

TCP/IP ... This stands for Transmission Control Protocol and Internet Protocol. It's really a "suite" of protocols and utilities. Think of it this way ... you have a 10000 page book. You need to mail it to someone and all you have are letter envelopes - a lot of them and a lot of stamps. When you address the outside of your envelope you're using IP - putting a source address and a destination address. Both of them have to be unique, or you will really confuse the post man. TCP is the envelope "wrapping" your information inside it. Maybe the type, size, and color of the envelope indicate what is inside it. Also, you can only put so many pages inside your envelopes - you can leave them empty, or put in up to 20 pages and still be able to seal your envelope. Your Post Office is going to be the "default gateway" determining if your letters are destined for someone outside your town or inside your town. Actually, your PC will do this for your data, but that's beside the point and doesn't fit into my analogy

Ok, so IP. You have to have a unique IP address on each machine on your network, if you're using this protocol. There is part of the address that describes your network, and part that describes which machine on your network. Another part of this address is the network mask, or subnet mask. Without getting really complicated, suffice it to say, a number other than 0 indicates which part of your IP address is the network and which part is the individual machine (host). All of your PC's on the same network must have the same network portion and the same network mask.

The default gateway ... this is a very special machine on your network usually a "router". Your PC will look at IP traffic and use your machines address and network mask to compare to the destination IP address and determine if the destination is local or not. If it's not, it will use the destination ethernet (MAC) address of the default gateway and send it there. The router (default gateway) will look at the destination IP address and consult some tables inside it, looking for a route to the destination. If it can't find it, it will send the traffic to its default gateway or gateway of last resort and so on and so on until some router knows how to get the traffic to the destination. Each time it goes through a router, the source and destination ethernet (MAC) addresses will be changed, but the IP source and destination address will remain the same.

TCP is Transmission Control Protocol and is used to set up a "session" with another machine. Once the two determine how they're going to talk and that they are and have exchanged greetings they will begin real communication. This is like 2 people on a phone - the phone call is "routed" according to unique addresses (phone numbers). The destination picks up the phone and says hello. The caller says, Hello ... (they're talking and acknowledging each other) and the rest of the call commences. TCP will contain a "port number" which is used to identify the type of traffic (the data) contained inside. This is like the callers saying, "This is Joe's Mom. Is Joe there?" Ok, so this might have a port number of 80 indicating a family call (port 80 is HTTP - Web Traffic). If Joe is there, presumably he'll be put on the phone - so his Port 80 is open and listening. If Joe didn't want to talk to his Mom - his port 80 would be blocked with a firewall - and the call would terminate - something like, "Joe's not here. Oh, OK. I'll call back later. OK, Goodbye. Goodbye". This is a TCP session tear down.

Firewalls ... Firewalls are special devices which strip off layers of traffic to examine the guts. A firewall will strip out the ethernet header, and the IP header (the portions where the MAC address and IP address are) and look at TCP and higher. Then it will consult its "rules" and determine what to do with the "envelope" it has just received from the post man. If the rule says to allow it, it's transmitted to the "inside" interface and allowed onto the network. If it's not allowed, it will get dropped in the trash can and instantly disintegrated. This is like when you go out to your mailbox and sort through your mail looking to see what you got and from whom and wether to send it back, accept it, or put it in the trash on your way back inside.

So, what you have are some hosts (PC's), their Network Interface Cards (NICs) with common protocols. They're connected via some kind of cable (or wireless) to a network device - a hub or a switch. If you want to "internetwork" you'll also have a router connected to other networks or other routers or to the internet.

This is the basics of Ethernet Networking using TCP/IP. If I've confused you or you need more explanation on any part of this, post up!

HTH

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good post.
good explanation. i feel like a lot of people can learn with your explanation the basics.

regards

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quote:

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Originally posted by pseudocyber
TCP/IP ... This stands for Transmission Control Protocol and Internet Protocol. It's really a "suite" of protocols and utilities. Think of it this way ... you have a 10000 page book. You need to mail it to someone and all you have are letter envelopes - a lot of them and a lot of stamps. When you address the outside of your envelope you're using IP - putting a source address and a destination address. Both of them have to be unique, or you will really confuse the post man. TCP is the envelope "wrapping" your information inside it. Maybe the type, size, and color of the envelope indicate what is inside it. Also, you can only put so many pages inside your envelopes - you can leave them empty, or put in up to 20 pages and still be able to seal your envelope. Your Post Office is going to be the "default gateway" determining if your letters are destined for someone outside your town or inside your town. Actually, your PC will do this for your data, but that's beside the point and doesn't fit into my analogy

---

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True - TCP is Transport Control Protocol. I was referring to a TCP three way handshake - when it will do a SYN a SYN ACK and an ACK. This is establishing the TCP "session". Perhaps a confusing choice of words.

The session layer, "synchronizes and sequences the dialogue and packets in a connection." - from Sybex Network+

I interpret this to mean that the session layer is getting the packets, possibly out of order, and gets them back in order for the higher layers.

The session layer also ensures the connection is maintained. I believe the session layer would tell the transport layer (TCP in our case) to terminate and TCP would then begin shutting down the connection by sending a FIN and waiting for an acknowledgement (skipping a few steps ...)

HTH!

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Thank you pseudocyber for the basics of networking. I am studying for my Network+ and every bit of information and explanation helps.

Thanks again!

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quote:

---

Originally posted by pseudocyber
True - TCP is Transport Control Protocol. I was referring to a TCP three way handshake - when it will do a SYN a SYN ACK and an ACK. This is establishing the TCP "session". Perhaps a confusing choice of words.

The session layer, "synchronizes and sequences the dialogue and packets in a connection." - from Sybex Network+

I interpret this to mean that the session layer is getting the packets, possibly out of order, and gets them back in order for the higher layers.

The session layer also ensures the connection is maintained. I believe the session layer would tell the transport layer (TCP in our case) to terminate and TCP would then begin shutting down the connection by sending a FIN and waiting for an acknowledgement (skipping a few steps ...)

HTH!

---

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Life, the only test that counts!!"

djm
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Good post ,although i am familiar with networking ,it was explained well.

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Nice summation pseudocyber! Any help is appreciated for those still working toward Network+.

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Thanks everyone.

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Excellent post pseudo. Although I'm gearing up to take the N+ in a couple of weeks, I'm sure there are alot of new comers who really appreciate the time you took to write this post. Very well written.

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