Layers 2 to 5
- Part 1 — Networks have layers
- Part 2 — The Physical Layer
- Part 3 — Layers 2, 3, 4, 5 (this article)
All these layers, and not enough time to write about them. I won’t bore you with re-hashed information when someone else has successfully written an excellent overview of the topic, so without further ado I present the rest of the layers.
Layer 2 — Ethernet and switching
Layer 3 — Routing
Layer 4 — Transports
Layer 5 — Applications
The application layer is important for a network consultant to know about, but it is beyond what I’m trying to do here. You will need to know how these applications work, what ports they use, whether they’re on TCP or UDP, whether they use multicast, or in some cases you may even need to know the internal workings of the protocols.
There are a few protocols specific to networking, and network management. You should definitely understand these, and have a good working knowledge of how to set them up on your network devices: SSH, TELNET, SNMP, TFTP.
At a minimum you should have a basic understanding of the most common internet application protocols:
- DHCP — RFC2131
- DNS — Various DNS RFCs
- FTP — RFC959
- SSL — Wikipedia Page on SSL
- HTTP — Various HTTP RFCs
- POP3 — RFC1939
- SMTP — RFC2821
- PoS — RFC3203
To learn these protocols, it helps to be a systems administrator. That way you can have a system online that you can interact with locally without the risk of crashing someone’s corporate FTP server. One thing you will notice with most of these protocols, is that you can test them directly using telnet.
Like this:
telnet SERVER PORT
telnet 10.10.10.10 25This opens a telnet session to a server running SMTP (port 25). An SMTP server will accept this connection, and it will reply within your telnet session, providing an opportunity to query the server and even compose your own email. The SMTP protocol works in this very way, using pseudo-English commands to create an email and submit it to the server — except these commands are usually automated by an email client like Mozilla Thunderbird, or Outlook Express.
To really learn these protocols, you need to use them and in this case my advice is to try Linux. Linux is free, and there is a huge support structure out there to help you setup these applications. By and large, you will be able to find free versions of all of these applications to test with.
[...] And here is a link to the original article that you were looking for! http://wozney.ca/2008/05/29/layers-2-to-5/ [...]