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 of the 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 the IP fragmentation and reassembly process which is defined in IP.