ICMP

While TCP and UDP differ dramatically-the former connection-oriented and the latter connectionless-both manage and modify packets in the classic sense with destination IP address, source IP address, destination port numbers, and source port numbers. A single session might be one packet or a series of packets.

On the other hand, sometimes applications are so simple that they're always connectionless and never need more than a single packet. These are called Internet Control Message Protocol (ICMP) applications. Although you won't see ICMP too often in configuration files and such, you do see the results. ICMP handles mundane issues such as disconnect messages (host unreachable) that applications use to let the other side of a session know what's happening.

Good old PING is one place where you'll see ICMP in action. PING is an ICMP application that works by sending a single ICMP packet called an echo request to an IP address you specify. All computers running TCP/IP (assuming there is no firewall involved) respond to echo requests with an echo reply.

NOTE A firewall is a device or software that filters all the packets between two computers (or groups of computers) and acts like a club bouncer deciding who gets in and who gets blocked.