Transcript IP: Routing and Subnetting

IP: Routing and Subnetting
Network Protocols and Standards
Autumn 2004-2005
Oct 28, 2004
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Issues in Addressing
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A large corporate/campus environment
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Large number of Local Area Networks
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If each physical network is assigned a network
number:
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Some with fewer than 256 hosts
Some with more than 256 hosts
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
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Solution: Provide the campus with a
single class B network
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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
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Consider a class B network
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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
R
R
R
Physical Network
(Subnet 2)
H
Physical Network
(Subnet 3)
R
Physical Network
(Subnet 1)
H
H
R
Physical Network
(Subnet 4)
H
H
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H
H
H
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Subnetting
Network 128.10.1.0
H1
Internet
R
128.10.1.1
H2
128.10.1.2
Network 128.10.2.0
R is not a Proxy ARP router!
H3
128.10.2.1
Subnet 1
Subnet 2
H4
128.10.2.2
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
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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
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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
Internet Part
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Local Part
Subnet
Host
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Subnetting
Examples: Class B IP address
Internet Part
16bits
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Subnet
Host
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 Mask
(32bits)
16bits
11111111
255.
11111111
255.
Internet Part of Address
11111111
255.
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16bits
11111111
255.
Subnet
Host
8bits
11111111
8bits
00000000
255.
0
Subnet
3bits
111
00000
224.
Host
13bits
00000000
0
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Subnetting
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It is recommended that sites use contiguous
subnet masks
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Avoid masks such as
11111111 11111111 11000010 11000000
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When choosing a subnet mask, balance:
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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
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Conventional routing table entry
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(network address, next hop address)
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Network address format is predetermined for a given
class (e.g., first 16 bits for class B addresses!)
With subnetting, routing table entry becomes
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(subnet mask, network address, next hop
address)
Then compare with network address field of
entries to find next hop address
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Subnet mask indicates the network address!
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Subnet Routing
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The use of mask generalizes the subnet routing algorithm to
handle all the special cases of the standard algorithm
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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:
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Individual host (Mask of all 1’s, Host IP address)
Default Route (Mask of all 0’s, network address all 0’s)
Class B network address (Mask of two octets of 1’s and two of 0’s)
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Subnet Routing
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Algorithm
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Extract destination IP (D) from datagram
Compute IP address of destination network N
If N matches any directly connected network address
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Else
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Send datagram over that network (obviously encapsulated in a
frame)
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
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Use of many IP network addresses for a
single organization
Example:
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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:
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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
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It requires each block to be a power of 2 and
uses bit mask to identify the size of the block
Example
Dotted decimal
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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?
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Answer: NO  CIDR (Classless Inter Domain Routing)
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Supernet Addressing
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In the router, the entry consists of:
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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
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Motivation
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Limited address space
Support for new applications
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Multimedia streams, for example
Security
Extensibility
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Features of IPv6
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Larger addresses
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Flexible header format
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Set of optional headers
Support for flow identification
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128 bit addresses
Needed in resource allocation for
multimedia streams
Provision for protocol extension
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