Extending the IP Addressing Scheme
LESSON 12

Outline:
Extending the Addressing Scheme

Address Extensions Schemes:

• Three extensions of the original IP address scheme were designed to allow a site to use a single IP network address for multiple physical networks and to handle the address shortage problem.
  • Transparent Gateways
  • Proxy ARP
  • Subnet Addressing/Routing
• A fourth was added that allowed multiple network addresses to be used for a single autonomous site.
  • Supernet Addressing/Routing
  
  
• Although these schemes have served to extend the lifetime of the current IP addresses, they have not eliminated the need to create a larger addressing scheme.

Transparent Gateways:

• Transparent Gateways:
  • Extend Class A IP addresses by arranging for a WAN to multiplex several host connections through a single host port.
  

• T: Other hosts and gateways do not know it exists.
  
• Ex: ARPANET => Class A address: 10.0.0.0
• Each Packet Switching Node (PSN) is assigned a unique combination of:
  • 10.P.U.I
• Where:
  • P: Specifies the port in the node at the destination PSN.
  • I: Specifies the destination PSN.
  • U: Not used for specification (10.2.5.37 and 10.2.9.37 are the same)
• Transparent gateways use U to decide which real host should receive a datagram.
• Problems caused by a transparent gateway are difficult to detect.
• Advantages:
  • Require fewer network addresses.
  • Supports load balancing.
• Disadvantages:
  • Only works well with Class A addresses (not Class C)
  • Cannot provide standard gateway services (echo request/reply, time-stamp, etc.)

Proxy ARP:

Proxy ARP (Promiscuous ARP or ARP hack):
• Allows the same IP address to be shared among multiple physical networks.
• Applies to networks that use ARP for binding IP and physical addresses.

• IP Spoofing safeguards cannot be used with Proxy ARP because they will generate many control messages.
• Routing in Proxy ARP gateways is based on location of the node and the node's IP address relative to the Proxy ARP gateway.
• Routing is NOT based on the NETID portion of the address, like normal routing.
• Advantage - when added, it does not disturb routing tables in other hosts and gateways.
• Disadvantage - does not generalize to more complex topologies.

Subnet Addressing and Routing:

• This is the most accepted IP extension method to date. It has been standardized by the IAB.
• Subnet addressing involves dividing the HOSTID part of an IP address into two sub-parts that identify:
  • A physical network (usually within an autonomous system)
  • A host on that network.
  

• Subnetting forms a more defined hierarchical address scheme with a corresponding hierarchical routing scheme.
• The subnetting standard allows flexibility in bit division, however once a selection has been made, all machines must honor it.
• Original:
  • NETID = Internet part
  • HOSTID = Local part.
• New Scheme:
  • Internet part.
  • Local Part = (Physical network subnet, HOSTID on that subnet).
  

• Subnet interpretation can be chosen independently for each physical network.
• The standard specifies that a site using subnet addressing must choose a 32-bit subnet mask for each network.
• It is recommended that sites use contiguous subnet masks and that all physical networks sharing the same IP address, use the same subnet mask.

Mask Representations:

• Binary mask representation is awkward and prone to errors:
  • 11101110111000001110110010100001
  
  
• Dotted-decimal notation:
  • 255.255.255.0
• 3-Tuple representation:
  {network-number, subnet-number, host-number}
  • (-1) implies all ones, so:
    • 255.255.255.0 = {-1, -1, 0}
• Disadvantage of 3-tuple representation is that it does not specify the number of bits used in each address part (number).

Subnet Routing:

• Subnet Routing:
  • Uses a modified IP routing algorithm that includes subnet masks as well as NETID and Next-hop addresses.
• A subnet routing table entry is made of
  {subnet mask, network address, next-hop address}
  
  

• When routing, for each packet, a bit-wise AND operation is performed on the:
  • 32-bit destination IP address
  • A 32-bit subnet mask of an entry in the routing table.
• The result is compared with the network address (NETID) of that table entry:
  • If a match is found, route the datagram to the next-hop address.
  • If they do not match, compare with all other entries until we find a match or report a routing error if a match is not found.
• ? How must subnet masks be assigned ? (Be conservative)
  ? How should subnet masks be propagated to other networks ?
  (Send an ICMP Subnet Mask Request or use a modern configuration protocol: DHCP)

Subnet Broadcasting:

• Broadcasting to all hosts on a subnet can be done similar to regular direct broadcasting using the mask:
  • {network, subnet, -1}
  • where -1 represents all ones for HOSTID.

Supernet Addressing:

• Supernet Addressing was invented in 1993 to further extend the 32-bit IP address lifetime since IPv6 (IPng) was not yet standardized or adopted.
• Subnet Addresses cannot prevent the eventual exhausting of Class B IP addresses.
• Supernetting (or Supernet Addressing
  • Is the opposite of subnet addressing in that it uses multiple Class C IP network addresses for a single organization instead of only one.
• Supernetting assigns a block of Class C addresses to an organization:
  • 1 Class B = 256 Class C addresses
    (3rd octet of Class B is commonly used for the subnet portion)
• Adopted for three reasons:
  1. IP does not divide network addresses into classes equally (only 17 thousand Class B vs. 2 million Class C network numbers).
  2. Only a small percentage of Class C addresses have been requested/assigned.
  3. The rate at which Class B addresses where assigned would've exhausted them in a few years.

Effect of Supernetting on Routing:

• Using supernetting creates the problem of now storing the dramatically increased amount of routing information needed.
• To prolong the depletion of available addresses and solve this routing problem, Classless Inter-Domain Routing was invented.

Classless Inter-Domain Routing
(CIDR):

• Classless Inter-Domain Routing (CIDR)
  • Used to handle this problem by conceptually collapsing a block of contiguous Class C addresses into a single routing table entry.
• A routing table entry consists of:
  • (Network Address, Count)
• (Network Address, Count) = 1 table entry:
  • Network Address - smallest network address in the block.
  • Count - total number of network addresses in the block.
• Entry (192.5.48.0, 3) =
  [192.5.48.0 |192.5.49.0 | 192.5.50.0]
• CIDR:
  • Does not restrict network numbers to Class C addresses
  • Does not use an integer count for the block size.
  • Requires each block of addresses to be a power of two
  • Uses bit masking to identify the block size.

Coordination of Address Allocation:

• To equally distribute the remaining available Class C addresses, they have been broken up into address groups.
• Each continent receives a block of addresses to be administered by a continent-level authority.
• Sub-authorities are delegated to disperse addresses within each country.
• All addresses were divided so that each address group differs by two numbers in the first octet.

  Multi-regional	192.0.0.0 - 193.255.255.255
  Europe	194.0.0.0 - 195.255.255.255
  Others	196.0.0.0 - 197.255.255.255
  North America	198.0.0.0 - 199.255.255.255
  Central/South America	200.0.0.0 - 201.255.255.255
  Pacific Rim	202.0.0.0 - 203.255.255.255
  Others	204.0.0.0 - 205.255.255.255
  Others	206.0.0.0 - 207.255.255.255

  Organizational Requirements		Assignment
  #	Fewer than
  1)	256 addresses	1 class C network
  2)	512 addresses	2 contiguous class C networks
  3)	1024 addresses	4 contiguous class C networks
  4)	2048 addresses	8 contiguous class C networks
  5)	4096 addresses	16 contiguous class C networks
  6)	8192 addresses	32 contiguous class C networks
  7)	16384 addresses	64 contiguous class C networks

CIDR Address Range Values:

• Two values are required to specify the range:
  1. Lowest address
  2. 32-bit mask (which operates like a subnet mask)
• Lowest - First address in the range
• Highest - Last valid address in range, usually broadcast.
• Mask - Similar to subnet masks
• Example: (2048 contiguous addresses starting at 234.170.168.0):
  • Lowest:
    (234.170.168.0)
    11101010 10101010 10101000 00000000

    Highest:

    (234.170.175.255)

    11101010 10101010 10101111 11111111

    CIDR Mask:

    (255.255.248.0)

    11111111 11111111 11111000 00000000

CIDR Router Functionality:

• Routers at a site with classless addresses must be changed to correctly route datagrams.
• Address Class Interpretation within the router must be disabled.
• To determine the correct destination, each entry in the routing table (pair of address and mask) and the routing software use a "longest-match" paradigm to select a route.
• A given block of addresses can be subdivided and separate routes can be setup for each subdivision.
• All nodes on a given network will be assigned addresses from the same fixed range.
• Hosts and routers that use supernetting need unconventional routing software that understands ranges of addresses.

Last Modification: (Sunday, August 25, 1996)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1996 Illusion Industries Inc.