The TCP/IP Protocol Family

DARPA: The Defense Advanced Research Project Agency

The United States Department of Defense began building a network in 1968 through its Advanced Research Projects Agency (ARPA). This network, called the ARPANET, was designed to meet a specific set of goals:

  • Share large-scale (..in 1968?) computer resources
  • Develop a reliable digital communications infrastructure

By 1975 the network consisted of only about 60 nodes and was, at that time, expanding at the rate of about one node every three months. The ARPANET grew, expanded, and the rest of the computing community joined in and, today, we have The Internet. The growth rate of the Internet has been exponential and the exact number of nodes is unknown. It’s probably in the hundreds of millions.

Originally, a Network Control Center was established through the company Bolt, Beranek, and Newman along with an additional Network Control Center at the Stanford Research Institute in Palo Alto, California. They used a DEC PDP-10 computer as the single, central controller for the ARPANET.

A family of communications protocols was developed to meet the objectives of the ARPANET. The original objectives continue to be met by these protocols, now known as the TCP/IP, or simply, the IP Protocols.

These objectives included the following (Compendium topics are linked):

  • Internet Protocol (IP)
    An addressing scheme to logically identify devices and to group them into networks
  • Address Resolution Protocol (ARP)
    The ability to resolve physical addresses when a logical (IP) address is known
  • Domain Name Service (DNS)
    A redundant storage system for network address information
  • Routing Information Protocol (RIP)
    The ability to re-route to an alternative path if a current path becomes unusable through router configuration
  • Internet Control Message Protocol (ICMP)
    The notification of error conditions on the network
  • User Datagram Protocol (UDP)
    A simple mechanism for exchanging data messages
  • Transmission Control Protocol (TCP)
    The reliable transfer of data as required
  • File Transfer Protocol (FTP)
  • Trivial File Transfer Protocol (TFTP)
    The ability to transfer files
  • Simple Main Transfer Protocol (SMTP)
    A mail messaging system
  • Telnet
    The ability to act as a terminal to a host computer

And, as the years went by, more additions were made to the original protocols, enhancing and expanding the original functionality. Here is a broad outline of some of these newer capabilities:

  • Open Shortest Path First (OSPF)
    Cisco’s Internet Gateway Routing Protocol (IGRP)
    Enhanced Internet Gateway Routing Protocol (EIGRP)

    Enhanced routing capabilities
  • Network Information Service (NIS)
    Expanded capabilities for storing network information other than addresses
  • Simple Network Management Protocol (SNMP)
    Remote router management
  • Network File System protocol (NFS)
    The ability to mount a remote file volume and treat it as if it were local
  • Remote Procedure Call protocol (RPC)
    A programming interface to facilitate the programming of network services
  • Boot Protocol (BOOTP)
    Dynamic Host Configuration Protocol (DHCP)

    Enhanced capabilities to provide bootstrap information for network clients
  • The Remote Monitoring MIB (RMON)
    The acquisition of network performance data

This section of the compendium discusses the broad spectrum of protocols under the umbrella of Internet Protocol. It is, by no means, complete – but we’re working on it. Refer to the NEWS section to find out what the most recent changes are to the Compendium.

Topics In This Discussion


Internet Protocol (IP)
Internet Protocol (IP) was originally designed to operate on top of Version 2 Ethernet. The Compendium has a separate section to discuss Ethernet. Various components of the IP protocol family were differentiated by Ethertype number. IP is assigned Ethertype 0800 hex.
When the IEEE developed the 802.3 standards for Ethernet they, essentially, replaced the Ethertype number with a Service Access Point identifier. It was necessary to include an option for embedding the original Ethertype inside a newer 802.3 frame in order to allow access to an IP Subnet. This is why there is a Sub-Network Access Protocol (SNAP) header in most IP frames that aren’t using Version 2 Ethernet.
IP operates at OSI Layer 3 and provides the routing function in an IP network. Each communicating device is assigned an IP address. The address identifies the network (which may be divided into sub-networks) and the host. The term “host” refers to any communicating device in an IP network. Originally the term referred to a central host computer. Today it includes any PC, printer, gateway, file server, or other device that has an IP address and talks on an IP network.

The discussion of IP begins with a description of the addressing scheme, progresses through the routing function, and then expands on the addressing concepts used to create sub-networks. Troubleshooting IP is the process of troubleshooting routing on the network.

  • IP Addressing
    This topic describes the binary nature of the IP address and the structure of the address fields.
  • IP Routing Functions
    This topic describes the way IP uses a routing table to make forwarding decisions.
  • Subnet Masking
    This topic explains the way IP addresses use bit fields to represent logical divisions in the network, called SubNetworks. Bits are assigned to identify the network portion of the address, the subnet portion, and the remainder are the host portion.
  • Address Classes
    This topic describes the fundamental “classes” of IP addresses; major address divisions defined by the standards. Three standard address masks (subnet masks) are used to differentiate the address classes.
  • Creating Subnets
    This topic details the mechanism whereby IP addresses represent network, subnetwork, and host by using bit fields. The process of configuring these bit fields (the ‘subnet mask’) is described.
  • Special Address Masks
    This topic talks about extending the idea of address classes to create ‘non-standard’ masks. The masks are ‘non-standard’ because the go beyond the original specification for address class. These types of masks are in common use today; hence, the ‘non-standard’, special address masks are, in fact, very ‘standard’ and typical.
  • Reserved Address List
    This topic describes and lists some IP address constructions that have been assigned specific meanings and, therefore, are not available for use as unique end-station addresses.
  • VLSM – Variable Length Subnet Masking
    This topic explains VLSM, the configuration of different subnet masks at different levels of the network tree topology.
  • Troubleshooting TCP/IP Networks
    This topic talks about troubleshooting methodology for IP-related problems. This is a section of the Compendium that we are working to constantly expand. We welcome your FEEDBACK.
  • IP Routing Tables
    This topic explains how a host (any communicating device) makes a forwarding decision by evaluating the contents of its routing table.
  • Physical Addresses
    This topic introduces the ARP (Address Resolution Protocol) and explains how it is used by IP to resolve a physical network interface card address from an IP address. A separate section of the compendium talks in detail about the ARP Protocol.
  • IP Type Of Service
    This topic details the meaning of the bits in the IP Type Of Service field in the IP header.
  • IP Fragmentation
    This topic details the IP fragmentation and reassembly process which is defined in IP.