Those documentations describe networking topics. Some are easy to read, but other are not : choose the one which corresponds to your feeling.
If your question is not answered in those docs, use a search engine.
If you know a good documentation which should be listed here, drop me a note.

## Network protocols

- rfc 826 : ARP
- rfc 903 : RARP
- rfc 791 : IP
- rfc 793 : TCP
- rfc 768 : UDP
- rfc 792 : ICMP
- TCP/IP tutorial
- Ethernet overview
- Ethernet tutorial
- Ethernet tutorial
- Ethernet tutorial
- IP overview
- TCP/IP part 1 part 2
- TCP/IP overview
- TCP/IP overview
- TCP/IP tutorial part 1 part 2
- IP primer
- Networking presentation
- Networking tutorials
- TCP/IP hacking

## Application protocols

- rfc 959 : FTP
- rfc 2068 : HTTP
- rfc 821 : SMTP
- rfc 977 : NNTP
- rfc 1036 : NNTP
- HTTP tutorial
- SMTP tutorial
- SMTP tutorial

## Sniffing and spoofing

- Sniffing faq
- Sniffer faq
- RAW IP faq
- Switched networks
- IP spoofing demystified
- IP spoofing with Linux
- IP spoofing attacks and hijacked terminal connections
- IP spoofing guides

## Miscellaneous

- Hacking dictionary