Transcript Global Internet

Global Internet
Computer Networks: Global Internet
Global Internet
NSFNET backbone
Stanford
ISU
BARRNET
MidNet
regional
Westnet
regional
■■■
regional
Berkeley
PARC
UNM
NCAR
UNL
KU
UA
Figure 4.24 The tree structure of the
Internet in 1990
Computer Networks: Global Internet
Global Internet
• Each provider network is regional and a
single autonomous system (AS)
• Major issues are:
– Scalability of routing
– Address utilization (running out of IP
addresses
• Hierarchy is used to improve scalability.
Computer Networks: Global Internet
Subnetting
• Assigning one network number per physical
network uses up IP address too fast!
• More network numbers also increases
forwarding table size.
• The idea is to take a single IP network number
and allocate the IP addresses with that network
number to several physical networks which are
referred to as subnets.
• The subnets need to be close to each other for
routing purposes.
Computer Networks: Global Internet
Subnetting
• The mechanism by which a single network
nubmer can be shared among multiple networks
involves configuring all the nodes on each
subnet with a subnet mask.
• The subnet mask enables introduction of a
single subnet number which provides for another
level of hierarchy into the IP address.
• All hosts on a given subnet are configured with
the same mask, i.e., there is one subnet mask
per subnet.
Computer Networks: Global Internet
Subnetting
Netw ork number
Host number
Class B address
111111111111111111111111
00000000
Subnet mask (255.255.255.0)
Netw ork number
Subnet ID
Host ID
Subnetted address
Figure 4.25 Subnet Addressing
Computer Networks: Global Internet
Subnetting
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.0
128.96.34.15
128.96.34.1
R1
H1
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.128
128.96.34.130
128.96.34.139
128.96.34.129
H3
R2
H2
128.96.33.1
128.96.33.14
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
Figure 4.26 An Example of Subnetting
Computer Networks: Global Internet
Classless Routing (CIDR)
• Classless interdomain routing (CIDR)
addresses:
– The growth of backbone routing tables
– The potential for 32-bit IP address space to be
exhausted
• The problem is with the Class B numbers (64K
addresses)
– Give out appropriate number of Class C addresses in
256 address chunks.
– But this increases routing table entries for a single
AS!
Computer Networks: Global Internet
Classless Routing (CIDR)
• CIDR helps us to aggregate routes by breaking
up rigid boundaries between classes.
• Hand out Class C addresses in contiguous
blocks by address.
• Make it so the addresses share a common prefix
=> allocate Class C networks as a power of 2.
• We need a protocol that understands these
rules, e.g., BGP!
• Network numbers are represented by
(length,value) where length is the length of the
prefix {similar to a mask}.
Computer Networks: Global Internet
Classless Routing (CIDR)
Corporation X
(11000000000001000001)
Border gatew ay
(advertises path to
11000000000001)
Regional netw ork
Corporation Y
(11000000000001000000)
Figure 4.27 Route Aggregation with CIDR
Computer Networks: Global Internet