IP: Routing and Subnetting

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IP Routing and Subnetting

• Network Protocols and Standards
• Autumn 2004-2005

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Routing IP Datagram

• Direct Delivery (i.e., not involving routers)
• Transmission of an IP datagram between two
  machines on a single physical network does not
  involve routers
• The sender encapsulates the datagram in a
  physical frame, binds the destination IP address
  to a physical hardware address (using ARP), and
  sends the resulting frame directly to the
  destination
• The two machines are known to be on the same
  network because they have the same network
  identifier
• Example
• A sends IP Datagram to B

Router
A
B
C
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Routing IP Datagram

• Indirect delivery (i.e. through intermediate
  routers)
• Host performs routing decisions based on routing
  table indicating next hop
• Next hop refers to next router IP address on
  this network, via which the destination is
  reached
• Routing decisions are made based on network
  prefixes (not full IP address)
• The sender encapsulates the datagram in a frame
  with the routers physical destination address
  (which is found by means of ARP).

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Direct and Indirect Routing
B wants to send packets to A and C!
Host A 204.240.18.10
Internet
204.240.18.1
Router
Direct Routing Packets sent directly using MAC
address of A
Indirect Routing Packets sent to the MAC
address of the router. At the IP level, B is the
source and C is the destination
Host B 204.240.18.20
Host C 36.14.0.200
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IP Routing Decisions
10.0.0.5
40.0.0.7
20.0.0.6
30.0.0.6
20.0.0.5
R3
R1
R2
30.0.0.7
Routing Table of R2
To Reach Hosts on Network Next Hop Address
20.0.0.0 Direct Delivery
30.0.0.0 Direct Delivery
10.0.0.0 20.0.0.5
40.0.0.0 30.0.0.7
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IP Routing Algorithm

• Router receives an IP datagram with network
  portion N and destination D
• If N is directly connected
• Transmit on that network
• Else If host specific entry for D exists
• Use next hop in that entry
• Else If route entry for N exists
• Use next hop in that entry
• Else If default route for next hop exists
• Use default route for next hop
• Else
• Declare error

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Routing Within Same Network

• Consider a small company with a single LAN to
  which a class C network address has been assigned
• The company is interested in adding another small
  physical network (connected to old network
  through a router) with a few hosts
• Question Could this company assign these hosts
  IP addresses from the same C class network? i.e.,
  could the two LANs share the same class C network
  address?

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Proxy ARP

• Used to allow two physical networks to share the
  same IP network prefix
• Router Rs table is configured manually to route
  between these two networks
• Router R answers ARP requests on each network for
  hosts on the other network, giving its own
  hardware address as the target address

Main Router
To Internet
Main Network
A
B
C
Router R
E
D
Hidden Network
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Proxy ARP

• Advantage of Proxy ARP Router
• Can be added without disturbing the routing table
  in other hosts or routers on that network
• Disadvantages
• Does not generalize to complex network topologies
  (does not scale)
• Does not support a reasonable form of routing.
  (relies on network managers to maintain tables of
  machines and addresses manually)
• Issues
• Several IP addresses map to the same physical
  address. How to distinguish between a legitimate
  Proxy ARP router and spoofing?

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Issues in Addressing

• A large corporate/campus environment
• Large number of Local Area Networks
• Some with fewer than 256 hosts
• Some with more than 256 hosts
• If each physical network is assigned a network
  number
• Immense administrative overhead to manage a large
  number of network addresses
• Routing tables in routers become extremely large
  (one entry for each physical network)
• Insufficient number of class B prefixes to cover
  medium sized networks (having more than 256 hosts)

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Subnetting

• Solution Provide the campus with a single class
  B network
• Give freedom to the campus network admin to
  allocate host numbers to hosts
• From outside, the whole campus is simply known by
  the class B network ID
• Inside, there may be a hierarchy that remains
  transparent to the outside world

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Subnetting

• Consider a class B network
• How to allocate host numbers to hosts?
• A single LAN is out of question
• If host numbers are assigned randomly, i.e.,
  without any hierarchy, the routers inside the
  network will have to deal with large tables one
  entry per host
• Thus, a hierarchical structure is required

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Subnetting
H
H
H
H
Physical Network (Subnet 3)
Physical Network (Subnet 1)
R
H
H
R
R
H
R
Physical Network (Subnet 4)
Physical Network (Subnet 2)
R
H
H
H
H
H
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Subnetting
Network 128.10.1.0
Subnet 1
H1
H2
Internet
R
128.10.1.1
128.10.1.2
Network 128.10.2.0
Subnet 2
H4
H3
R is not a Proxy ARP router!
128.10.2.2
128.10.2.1
H1 wants to send an IP datagram to H3 Old
addressing dictates it is a direct
delivery With subnetting, it may become
indirect
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Subnetting

• We previously divided IP addresses in a network
  portion and a host portion
• More generally, think of a 32-bit IP address as
  having an Internet part and a Local part
• Internet part of the IP address identifies a site
  (possibly with many physical networks)
• The local portion identifies a physical network
  and host at that site

Internet Part
Local Part
Internet Part
Subnet
Host
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Subnetting
Examples Class B IP address
Internet Part
Subnet
Host
16bits 8bits 8bits
Internet Part
Subnet
Host
16bits 3bits 13bits
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Subnet Implementation
Subnet Mask Specifies the bits of the IP
address used to identify the subnet
Internet Part of Address
Subnet
Host
16bits 8bits 8bits
11111111 11111111 11111111 00000000
Subnet Mask (32bits)
255. 255. 255. 0
Internet Part of Address
Subnet
Host
16bits 3bits 13bits 11111111
11111111 111 00000 00000000
255. 255. 224. 0
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Subnetting

• It is recommended that sites use contiguous
  subnet masks
• Avoid masks such as
• 11111111 11111111 11000010 11000000
• When choosing a subnet mask, balance
• Size of networks
• Number of networks
• Expected growth
• Ease of maintenance
• It is possible to use different masks in
  different parts of the network

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Subnet Routing

• Conventional routing table entry
• (network address, next hop address)
• Network address format is predetermined for a
  given class (e.g., first 16 bits for class B
  addresses!)
• With subnetting, routing table entry becomes
• (subnet mask, network address, next hop address)
• Then compare with network address field of
  entries to find next hop address
• Subnet mask indicates the network address!

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Subnet Routing

• The use of mask generalizes the subnet routing
  algorithm to handle all the special cases of the
  standard algorithm
• Routes to individual hosts
• Default route
• Routes to directly connected networks
• Routes to conventional networks (that do not use
  subnet addressing)
• Merely combine the 32-bit mask field with the
  32-bit IP address
• Example To install a route for
• Individual host (Mask of all 1s, Host IP
  address)
• Default Route (Mask of all 0s, network address
  all 0s)
• Class B network address (Mask of two octets of
  1s and two of 0s)

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Subnet Routing

• Algorithm
• Extract destination IP (D) from datagram
• Compute IP address of destination network N
• If N matches any directly connected network
  address
• Send datagram over that network (obviously
  encapsulated in a frame)
• Else
• For each entry in the routing table, do
• N bitwise-AND of D and subnet mask
• If N equals the network address field of the
  entry, then route the datagram to the specified
  next hop

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Supernet Addressing

• Use of many IP network addresses for a single
  organization
• Example
• To conserve class B addresses, issue multiple
  class C address to the same organization
• Issue increase in the number of entries in the
  routing table
• Solutions
• Collapse a block of contiguous class C address
  into the pair (network address, count) where
  network address is the smallest number in the
  block

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Supernet Addressing

• It requires each block to be a power of 2 and
  uses bit mask to identify the size of the block
• Example
• Dotted decimal 32-bit binary equivalent
• Lowest 234.170.168.0 11101010 10101010 10101000
  00000000
• Highest 234.170.175.255 11101010 10101010
  10101111 11111111
• A block of 2048 addresses
• 32-bit mask is 11111111 11111111 11111000
  00000000
• Do we really need address classes when we have
  masks?
• Answer NO ? CIDR (Classless Inter Domain Routing)

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Supernet Addressing

• In the router, the entry consists of
• The lowest address and the 32-bit mask
• A block of addresses can be subdivided, and
  separate route can be entered for each
  subdivision
• When looking up a route, the routing software
  uses a longest-match paradigm to select a route

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IPv6

• Motivation
• Limited address space
• Support for new applications
• Multimedia streams, for example
• Security
• Extensibility

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Features of IPv6

• Larger addresses
• 128 bit addresses
• Flexible header format
• Set of optional headers
• Support for flow identification
• Needed in resource allocation for multimedia
  streams
• Provision for protocol extension