Some useful networking resources.
http://www.routeralley.com/ra/docs/tcp_udp.pdf TCP and UDP reference.
http://www.razorpoint.com/PDF/Rz.PortsList.pdf TCP/IP Ports listing. This is very useful for reference.
http://www.gasmi.net/tcp.php Another TCP ports listing in HTML.
http://www.hardwaresecrets.com/article/431 An introduction to the OSI model and how the TCP/IP Protocol works.
http://www.protocols.com/pbook/tcpip1.htm A definition of common TCP/IP names.
Intro to TCP/IP.
TCP/IP and subnet masking explained.
Kali Linux pentesting distribution: http://www.kali.org/downloads/.
Networking cheat sheets.
IEEE 802.11 WLAN cheat sheet: http://media.packetlife.net/media/library/4/IEEE_802.11_WLAN.pdf.
Cisco IOS Internal Routing Protocols cheat sheet: http://packetlife.net/posters/IOS_Interior_Routing_Protocols.pdf.
IPsec cheat sheet: http://media.packetlife.net/media/library/6/IPsec.pdf.
IPv4 Multicast cheat sheet: http://media.packetlife.net/media/library/7/IPv4_Multicast.pdf.
IPv6 cheat sheet: http://media.packetlife.net/media/library/8/IPv6.pdf.