Data is sent in limited-size packets, which are handled indivdually for transmission and delivery ("packet-switched" vs. "connection-oriented")
Packet delivery is not reliable
Each packet contains a source address and a destination address used to route the packet
Addresses are really just 32-bit numbers, but are typically written as a sequence of four bytes expressed in decimal, so 128.223.32.56 is really 10000000110111110010000000111000
Understanding the binary addresses will be useful when we talk about netmasks for routing
IP isn't really used by itself; it's just the underlying common structure of the other protocols used in TCP/IP networking
IP header (common to later protocols):
This and later diagrams taken from the TCP/IP Illustrated Postcript pocket guide.