rfc1812.txt

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   router.  Many older Internet documents refer to routers as gateways.

   Historically, routers have been realized with packet-switching
   software executing on a general-purpose CPU.  However, as custom
   hardware development becomes cheaper and as higher throughput is
   required, special purpose hardware is becoming increasingly common.
   This specification applies to routers regardless of how they are
   implemented.

   A router connects to two or more logical interfaces, represented by
   IP subnets or unnumbered point to point lines (discussed in section
   [2.2.7]).  Thus, it has at least one physical interface.  Forwarding
   an IP datagram generally requires the router to choose the address
   and relevant interface of the next-hop router or (for the final hop)
   the destination host.  This choice, called relaying or forwarding
   depends upon a route database within the router.  The route database
   is also called a routing table or forwarding table.  The term
   "router" derives from the process of building this route database;
   routing protocols and configuration interact in a process called
   routing.

   The routing database should be maintained dynamically to reflect the
   current topology of the Internet system.  A router normally
   accomplishes this by participating in distributed routing and
   reachability algorithms with other routers.

   Routers provide datagram transport only, and they seek to minimize
   the state information necessary to sustain this service in the
   interest of routing flexibility and robustness.

   Packet switching devices may also operate at the Link Layer; such
   devices are usually called bridges.  Network segments that are
   connected by bridges share the same IP network prefix forming a
   single IP subnet.  These other devices are outside the scope of this



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   document.

2.2.4 Autonomous Systems

   An Autonomous System (AS) is a connected segment of a network
   topology that consists of a collection of subnetworks (with hosts
   attached) interconnected by a set of routes.  The subnetworks and the
   routers are expected to be under the control of a single operations
   and maintenance (O&M) organization.  Within an AS routers may use one
   or more interior routing protocols, and sometimes several sets of
   metrics.  An AS is expected to present to other ASs an appearence of
   a coherent interior routing plan, and a consistent picture of the
   destinations reachable through the AS.  An AS is identified by an
   Autonomous System number.


   The concept of an AS plays an important role in the Internet routing
   (see Section 7.1).

2.2.5 Addressing Architecture

   An IP datagram carries 32-bit source and destination addresses, each
   of which is partitioned into two parts - a constituent network prefix
   and a host number on that network.  Symbolically:

      IP-address ::= { <Network-prefix>, <Host-number> }

   To finally deliver the datagram, the last router in its path must map
   the Host-number (or rest) part of an IP address to the host's Link
   Layer address.

2.2.5.1 Classical IP Addressing Architecture

   Although well documented elsewhere [INTERNET:2], it is useful to
   describe the historical use of the network prefix.  The language
   developed to describe it is used in this and other documents and
   permeates the thinking behind many protocols.

   The simplest classical network prefix is the Class A, B, C, D, or E
   network prefix.  These address ranges are discriminated by observing
   the values of the most significant bits of the address, and break the
   address into simple prefix and host number fields.  This is described
   in [INTERNET:18].  In short, the classification is:

        0xxx - Class A - general purpose unicast addresses with standard
        8 bit prefix
        10xx - Class B - general purpose unicast addresses with standard
        16 bit prefix



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        110x - Class C - general purpose unicast addresses with standard
        24 bit prefix
        1110 - Class D - IP Multicast Addresses - 28 bit prefix, non-
        aggregatable
        1111 - Class E - reserved for experimental use

   This simple notion has been extended by the concept of subnets.
   These were introduced to allow arbitrary complexity of interconnected
   LAN structures within an organization, while insulating the Internet
   system against explosive growth in assigned network prefixes and
   routing complexity.  Subnets provide a multi-level hierarchical
   routing structure for the Internet system.  The subnet extension,
   described in [INTERNET:2], is a required part of the Internet
   architecture.  The basic idea is to partition the <Host-number> field
   into two parts: a subnet number, and a true host number on that
   subnet:

      IP-address ::=
        { <Network-number>, <Subnet-number>, <Host-number> }

   The interconnected physical networks within an organization use the
   same network prefix but different subnet numbers.  The distinction
   between the subnets of such a subnetted network is not normally
   visible outside of that network.  Thus, routing in the rest of the
   Internet uses only the <Network-prefix> part of the IP destination
   address.  Routers outside the network treat <Network-prefix> and
   <Host-number> together as an uninterpreted rest part of the 32-bit IP
   address.  Within the subnetted network, the routers use the extended
   network prefix:

      { <Network-number>, <Subnet-number> }

   The bit positions containing this extended network number have
   historically been indicated by a 32-bit mask called the subnet mask.
   The <Subnet-number> bits SHOULD be contiguous and fall between the
   <Network-number> and the <Host-number> fields.  More up to date
   protocols do not refer to a subnet mask, but to a prefix length; the
   "prefix" portion of an address is that which would be selected by a
   subnet mask whose most significant bits are all ones and the rest are
   zeroes.  The length of the prefix equals the number of ones in the
   subnet mask.  This document assumes that all subnet masks are
   expressible as prefix lengths.

   The inventors of the subnet mechanism presumed that each piece of an
   organization's network would have only a single subnet number.  In
   practice, it has often proven necessary or useful to have several
   subnets share a single physical cable.  For this reason, routers
   should be capable of configuring multiple subnets on the same



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   physical interfaces, and treat them (from a routing or forwarding
   perspective) as though they were distinct physical interfaces.

2.2.5.2 Classless Inter Domain Routing (CIDR)

   The explosive growth of the Internet has forced a review of address
   assignment policies.  The traditional uses of general purpose (Class
   A, B, and C) networks have been modified to achieve better use of
   IP's 32-bit address space.  Classless Inter Domain Routing (CIDR)
   [INTERNET:15] is a method currently being deployed in the Internet
   backbones to achieve this added efficiency.  CIDR depends on
   deploying and routing to arbitrarily sized networks.  In this model,
   hosts and routers make no assumptions about the use of addressing in
   the internet.  The Class D (IP Multicast) and Class E (Experimental)
   address spaces are preserved, although this is primarily an
   assignment policy.

   By definition, CIDR comprises three elements:

     o topologically significant address assignment,
     o routing protocols that are capable of aggregating network layer
        reachability information, and
     o consistent forwarding algorithm ("longest match").

   The use of networks and subnets is now historical, although the
   language used to describe them remains in current use.  They have
   been replaced by the more tractable concept of a network prefix.  A
   network prefix is, by definition, a contiguous set of bits at the
   more significant end of the address that defines a set of systems;
   host numbers select among those systems.  There is no requirement
   that all the internet use network prefixes uniformly.  To collapse
   routing information, it is useful to divide the internet into
   addressing domains.  Within such a domain, detailed information is
   available about constituent networks; outside it, only the common
   network prefix is advertised.

   The classical IP addressing architecture used addresses and subnet
   masks to discriminate the host number from the network prefix.  With
   network prefixes, it is sufficient to indicate the number of bits in
   the prefix.  Both representations are in common use.  Architecturally
   correct subnet masks are capable of being represented using the
   prefix length description.  They comprise that subset of all possible
   bits patterns that have

     o a contiguous string of ones at the more significant end,
     o a contiguous string of zeros at the less significant end, and
     o no intervening bits.




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   Routers SHOULD always treat a route as a network prefix, and SHOULD
   reject configuration and routing information inconsistent with that
   model.

      IP-address ::= { <Network-prefix>, <Host-number> }

   An effect of the use of CIDR is that the set of destinations
   associated with address prefixes in the routing table may exhibit
   subset relationship.  A route describing a smaller set of
   destinations (a longer prefix) is said to be more specific than a
   route describing a larger set of destinations (a shorter prefix);
   similarly, a route describing a larger set of destinations (a shorter
   prefix) is said to be less specific than a route describing a smaller
   set of destinations (a longer prefix).  Routers must use the most
   specific matching route (the longest matching network prefix) when
   forwarding traffic.

2.2.6 IP Multicasting

   IP multicasting is an extension of Link Layer multicast to IP
   internets.  Using IP multicasts, a single datagram can be addressed
   to multiple hosts without sending it to all.  In the extended case,
   these hosts may reside in different address domains.  This collection
   of hosts is called a multicast group.  Each multicast group is
   represented as a Class D IP address.  An IP datagram sent to the
   group is to be delivered to each group member with the same best-
   effort delivery as that provided for unicast IP traffic.  The sender
   of the datagram does not itself need to be a member of the
   destination group.

   The semantics of IP multicast group membership are defined in
   [INTERNET:4].  That document describes how hosts and routers join and
   leave multicast groups.  It also defines a protocol, the Internet
   Group Management Protocol (IGMP), that monitors IP multicast group
   membership.

   Forwarding of IP multicast datagrams is accomplished either through
   static routing information or via a multicast routing protocol.
   Devices that forward IP multicast datagrams are called multicast
   routers.  They may or may not also forward IP unicasts.  Multicast
   datagrams are forwarded on the basis of both their source and
   destination addresses.  Forwarding of IP multicast packets is
   described in more detail in Section [5.2.1].  Appendix D discusses
   multicast routing protocols.







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2.2.7 Unnumbered Lines and Networks Prefixes

   Traditionally, each network interface on an IP host or rout