IP Basics

Overview
The Internet is a network of networks. It is built on IP – the Internet Protocol.
IP is a Network Level protocol on the OSI stack
An IP packet is carried in the payload part of the datalink layer protocol (typically Ethernet).
The IP packet also has a header and a payload
The IP header includes Source IP, Destination IP, Payload protocol type and other important control information.
Higher level (Transport level) protocols commonly used with IP are UPD, TCP and ICMP.
IP is a best-effort packet delivery system. This means that (a) it delivers each packet individually and independently, (b) it offers no guarantee that a packet will be delivered and (c) it offers no mechanism for monitoring the success or failure of a packet delivery.
Key concepts





IP: Internet Protocol

UDP: User Datagram Protocol

TCP: Transport Control Protocol

ICMP: Internet Control and Management Protocol



Important commands





ipconfig (Windows NT / Windows 98)

Use ipconfig /all to see the complete IP configuration and the MAC address for any device.


winipcfg (Windows 95)

Use winipcfg to see the complete IP configuration and the MAC address for any device.


ifconfig (UNIX)

Use ifconfig -au to see the complete IP configuration and the MAC address for all devices on the host.


ping (UNIX and Windows)

Use ping to determine whether a host with a given IP address is active somewhere on the Internet.


tcpdump (UNIX)

Use tcpdump –e host and port to view all packets meeting the specified parameters that are traveling on the network.

NOTE: You leave out the –e if you are not interested in datalink layer information and you can leave out port if you are interested in traffic on all ports.



Tips and tricks
Use ping –t under Windows to do a continuous ping.
Use ping –w under Windows to extend timeout in milliseconds to wait for each reply. 3000 is a safe bet on the Internet.
If the address you are pinging is on a different IP subnet, then there will be no entry for it in the ARP table. However, there should be an ARP entry for the nearest router.
See http://www.whatis.com/ipaddres.htm for a very good description.