IP Addressing

1
IP Addressing Forwarding

• Discussion
• Dilip Antony Joseph

2
Agenda

• Understand classful and classless addressing
• Understand the meaning of A.B.C.D/X notation
• Illustrate how a big IP address range is split
  amongst different organizations
• Figure out how routers use the forwarding table

3
Rest of the World
8 is the number of bits in the network number
217 (131k) hosts
18.220.0.0/15
221 (2.09m) hosts
IP packet destined to 18.221.5.123 arrives at Bay
Area router
18.96.0.0/11
15 1s for Berkeleys 15 bit long network
no 11111111.11111110.00000000.00000000 255 .
254 . 0 . 0
Match Send pkt along R1
255.254.0.0
AND
18.220.0.0
18.191.0.0/16

• Where do these packets go?
• 18.191.42.51 (Stanford)
• 18.107.26.21 (Comcast)
• 18.230.53.1 (No match, so default)

Practice Exercise Design the subnet masks for
the internal networks for UCB, Stanford and
Comcast. Fill up the forwarding tables for the
three routers. When choosing the length of the
network number, minimize wastage of IP
addresses.You can choose any random network
number. Also come up with IP addresses for two
hosts within each organization, i.e. IP addresses
that will be routed there. Note that there is no
fixed answer to these questions. If you have any
doubts or find errors in the above example,
please email Dilip.
216 (65k) hosts
4
Notes

• All addresses in the examples are fictional. They
  do not correspond to the Bay Area. For example,
  18.0.0.0/8 is MITs address range.