Computer Networks (CS3623)

doctorheavenlyNetworking and Communications

Oct 24, 2013 (3 years and 11 months ago)

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Computer Networks

(CS3623)

#
12

|

Global Internet #1

Global Internet


not just a random interconnection of networks


it interconnects many different organizations


Internet in the 1990







“end user” sites


“service provider”
networks


“backbone”

Autonomous System


each provider and end user


an
administratively
independent

entity


significant consequences on
routing



different ideas about :


the best routing protocol to use


how metrics should be assigned


each provider’s network = 1 AS


AS ≈ a network that is administered
independently of other ASs

Scaling issues


the scalability of routing


to minimize the number of network numbers that
get carried around in routing protocols and stored
in the routing tables of routers.


address utilization


making sure that the IP address space does not
get consumed too quickly

Agenda

1.
Subnetting

2.
Supernetting

/ Classless
Interdomain

Routing (CIDR)


3.
Interdomain

Routing: BGP

4.
IPv6

1.
SUBNETTING

Addressing Problem


IP@ = Net@ + Host@


Cases:


A network with two nodes


class C network
address


wastes 253 addresses


A network with 257 hosts


class B


wastes
over 64,000 addresses



Assigning one network number per physical
network uses up the IP address much faster
than we would like


Simple way to reduce the total number of
network numbers that are assigned.



The idea: take a single IP network number and
allocate the IP addresses with that network
number to several physical networks (
subnets
)

Subnetting


At a distant point in the Internet, they will
look like a single network (having only one
network number)



Exp: large campus that has many physical
networks


To reach any subnet, all you have to know is the
point where the campus connects to the Internet

The subnet should be ‘close’ to
each other

Sharing a single network number
among multiple networks


configuring all the nodes on each subnet with
a
subnet mask


subnet mask


subnet number


all hosts on the same physical network will have
the same subnet number


hosts may be on different physical networks but
share a single network number

Example: share a single class B


128.96.0.0

IF

(
own_subnet_mask

AND
target_IP_addr
) =
own_subnet

THEN


send directly

ELSE


send to default router

When host want to send a packet...

Forwarding table for R1

*)

There is [always] a
default route entry

Forwarding Algorithm

D = destination IP address

for each forwarding table entry (
SubnetNumber
,
SubnetMask
,
NextHop
)

D1 =
SubnetMask

& D

if D1 =
SubnetNumber


if
NextHop

is an interface



deliver datagram directly to destination


else



deliver datagram to
NextHop

(a router)


Subnet mask need not to align with a byte
boundary

e.g. 255.255.255.128


It is not necessary for all the 1s in the subnet
mask to be continous


Exp: 255.255.255.1


Not recommended in practice


We can put multiple subnets on a single physical
network


Not all parts of the Internet see exactly the same
routing information

Some notes on subnetting

2.
SUPERNETTING

(CIDR)

Two scaling concerns in the Internet

1.
the growth of backbone routing tables


as more and more network numbers need to be
stored in them


2.
the potential for the 32
-
bit IP address space
to be exhausted


before the 4 billionth host is attached to the
Internet.


Cause: address assignment inefficiency


Supernetting


Aggregate routes


lets us use a single entry
in a forwarding table to tell us how to reach a
lot of different networks


Exp:


AS with 16 class C;
contiguous
network numbers


192.4.16


192.4.31


share common prefix
(11000000 00000100 0001)


20 bits network number


192.4.16/20

Classless Interdomain Routing (CIDR)

Routing protocol for CIDR


“classless”


it must understand that a
network number may be of any length


Example: BGP
-
4



Network number = (length, value)

Route aggregation with CIDR



Prefixes: any length


Possible: overlap prefixes in the forwarding
table


In the forwarding table:


171.69/16


171.69.10/24


A packet destined to 171.69.10.5


Matches both prefixes


Use the
longest match


171.69.10


A packet destined to 171.69.20.5


Match only 171.69

IP Forwarding with CIDR