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Glossary

This glossary provides definitions of key terms used in RFC 791 (Internet Protocol).

A​

Address: A fixed-length identifier (32 bits in IPv4) that uniquely identifies a host in the internet system. An IP address consists of a network portion and a host portion.

ARPANET: Advanced Research Projects Agency Network. One of the first packet-switching networks and a predecessor to the modern Internet.

C​

Catenet: A system of interconnected packet-switched computer communication networks. The term was used to describe what is now known as the Internet.

Checksum: A 16-bit value computed from the IP header used to detect errors in transmission. The checksum is recalculated and verified at each point where the header is processed.

D​

Datagram: A self-contained, independent entity of data carrying sufficient information to be routed from source to destination without reliance on earlier exchanges. The basic unit of data transmission in IP.

Destination Address: The 32-bit IP address of the intended recipient of the datagram.

Don't Fragment (DF): A flag bit in the IP header that, when set, indicates the datagram must not be fragmented. If fragmentation is required and DF is set, the datagram is discarded.

F​

Fragment: A portion of an IP datagram. Fragmentation occurs when a datagram must traverse a network with an MTU smaller than the datagram size.

Fragment Offset: A 13-bit field indicating where in the original datagram this fragment belongs, measured in units of 8 octets.

Fragmentation: The process of breaking a large IP datagram into smaller pieces (fragments) for transmission across a network with a smaller Maximum Transmission Unit (MTU).

G​

Gateway: A device that forwards IP datagrams between networks. In modern terminology, this is typically called a "router."

Gateway to Gateway Protocol (GGP): A protocol used by gateways to exchange routing information and coordinate internet control information.

H​

Header: The control information prepended to data. The IP header contains addressing, control, and other information necessary for routing and delivery.

Header Checksum: A field in the IP header containing a checksum computed over the header fields. Used to detect corruption of the header during transmission.

Host: An end system that originates or receives IP datagrams. Distinguished from gateways, which forward datagrams.

I​

ICMP (Internet Control Message Protocol): A protocol used for error reporting and diagnostic functions in IP networks. ICMP messages are carried as IP datagrams.

Identification: A 16-bit field in the IP header used to identify fragments of a particular datagram.

IHL (Internet Header Length): A 4-bit field specifying the length of the IP header in 32-bit words. Minimum value is 5 (20 bytes).

IMP (Interface Message Processor): The packet switches used in the ARPANET. Predecessors to modern routers.

Internet Address: See Address.

Internet Datagram: See Datagram.

Internet Module: The software component that implements the Internet Protocol.

L​

Local Network: The physical network to which a host or gateway is directly attached. Examples include Ethernet, Token Ring, or ARPANET.

Local Network Interface (LNI): The interface between the IP module and the local network protocol.

M​

More Fragments (MF): A flag bit in the IP header that, when set, indicates more fragments of the datagram follow. When clear, indicates this is the last (or only) fragment.

MTU (Maximum Transmission Unit): The largest packet size that can be transmitted over a particular network without fragmentation.

O​

Octet: A unit of 8 bits. Equivalent to a byte on most modern systems.

Options: Variable-length fields in the IP header used for control functions such as security, source routing, route recording, and timestamping.

P​

Padding: Zero bits added to the IP header to ensure it ends on a 32-bit boundary when options are present.

Precedence: A 3-bit field in the Type of Service that indicates the priority of the datagram.

Protocol: An 8-bit field in the IP header indicating the next level protocol (e.g., TCP, UDP, ICMP) used in the data portion of the datagram.

R​

Reassembly: The process of reconstructing the original datagram from its fragments at the destination.

Route: The path that a datagram takes through the network from source to destination.

Routing: The process of selecting a path for transmission of a datagram through interconnected networks.

S​

Source Address: The 32-bit IP address of the sender of the datagram.

Source Routing: An IP option that allows the source to specify the route a datagram should take through the network. Can be loose (some hops specified) or strict (all hops specified).

T​

TCP (Transmission Control Protocol): A connection-oriented, reliable transport protocol that operates above IP.

Time to Live (TTL): An 8-bit field in the IP header indicating the maximum time (in seconds) or number of hops the datagram may exist in the internet system. Decremented at each hop.

Total Length: A 16-bit field indicating the total length of the IP datagram (header plus data) in octets.

Type of Service (TOS): An 8-bit field in the IP header used to indicate the desired quality of service for the datagram (precedence, delay, throughput, reliability).

U​

UDP (User Datagram Protocol): A connectionless transport protocol that operates above IP.

Key Concepts​

Address Classes​

IPv4 addresses are divided into classes based on the high-order bits:

  • Class A: 0.0.0.0 - 127.255.255.255 (high-order bit is 0)
  • Class B: 128.0.0.0 - 191.255.255.255 (high-order bits are 10)
  • Class C: 192.0.0.0 - 223.255.255.255 (high-order bits are 110)
  • Class D: 224.0.0.0 - 239.255.255.255 (high-order bits are 1110, multicast)
  • Class E: 240.0.0.0 - 255.255.255.255 (high-order bits are 1111, reserved)

Protocol Numbers​

Common values for the Protocol field:

  • 1 = ICMP (Internet Control Message Protocol)
  • 6 = TCP (Transmission Control Protocol)
  • 17 = UDP (User Datagram Protocol)

Precedence Levels​

Values for the Precedence field in Type of Service:

  • 111 = Network Control
  • 110 = Internetwork Control
  • 101 = CRITIC/ECP
  • 100 = Flash Override
  • 011 = Flash
  • 010 = Immediate
  • 001 = Priority
  • 000 = Routine

Note: This glossary is based on RFC 791 (September 1981). Some terms and concepts have evolved since the original specification.

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