What Is TCP/IP?

TCP/IP is shorthand for a suite of protocols that run on top of IP. IP is the Internet Protocol, and TCP is the most important protocol that runs on top of IP. Any application that can communicate over the Internet is using IP, and these days most internal networks are also based on TCP/IP.

Protocols that run on top of IP include: TCP, UDP and ICMP. Most TCP/IP implementations support all three of these protocols. We’ll talk more about them later.

Protocols that run underneath IP include: SLIP and PPP. These protocols allow IP to run across telecommunications lines.

TCP/IP protocols work together to break data into packets that can be routed efficiently by the network. In addition to the data, packets contain addressing, sequencing, and error checking information. This allows TCP/IP to accurately reconstruct the data at the other end.

Here’s an analogy of what TCP/IP does. Say you’re moving across the country. You pack your boxes and put your new address on them. The moving company picks them up, makes a list of the boxes, and ships them across the country using the most efficient route. That might even mean putting different boxes on different trucks. When the boxes arrive at your new home, you check the list to make sure everything has arrived (and in good shape), and then you unpack the boxes and “reassemble” your house.

  • A suite of protocols
  • Rules that dictate how packets of information are sent across
    • multiple networks
    • Addressing
    • Error checking


Let’s start with IP, the Internet Protocol.

Every computer on the Internet has at least one address that uniquely identifies it from all other computers on the Internet (aptly called it’s IP address!). When you send or receive data—say an email message or web page—the message gets divided into little chunks called packets or data grams. Each of these packets contains both the source IP address and the destination IP address.

IP looks at the destination address to decide what to do next. If the destination is on the local network, IP delivers the packet directly. If the destination is not on the local network, then IP passes the packet to a gateway—usually a router.

Computers usually have a single default gateway. Routers frequently have several gateways from which to choose. A packet may get passed through several gateways before reaching one that is on a local network with the destination.

Along the way, any router may break the IP packet into several smaller packets based on transmission medium. For example, Ethernet usually allows packets of up to 1500 bytes, but it is not uncommon for modem-based PPP connections to only allow packets of 256 bytes. The last system in the chain (the destination) reassembles the original IP packet.

TCP/IP Transport Layer

What Is TCP/IP?
  • 21 FTP—File Transfer Protocol
  • 23 Telnet
  • 25 SMTP—Simple Mail Transfer Protocol
  • 37 Time
  • 69 TFTP—Trivial File Transfer Protocol
  • 79 Finger
  • 103 X400
  • 161 SNMP—Simple Network Management Protocol
  • 162 SNMPTRAP

After TCP/IP was invented and deployed, the OSI layered network model was accepted as a standard. OSI neatly divides network protocols into seven layers; the bottom four layers are shown in this diagram. The idea was that TCP/IP was an interesting experiment, but that it would be replaced by protocols based on the OSI model.

As it turned out, TCP/IP grew like wildfire, and OSI-based protocols only caught on in certain segments of the manufacturing community. These days, while everyone uses TCP/IP, it is common to use the OSI vocabulary.

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