Routing

1
Routing
2
Classless Inter-Domain Routing

• Classful addressing scheme wasteful
• IP address space exhaustion
• A class B net allocated enough for 65K hosts
• Even if only 2K hosts in that network
• Solution CIDR
• Eliminate class distinction
• No A,B,C
• Keep multicast class D

3
Classless Addressing

• Addresses allocated in blocks
• Number of addresses assigned always power of 2,
  and always on the boundary. That is, if 2048
  addresses, it will start with some address with
  all lower 11 bits being 0.
• Network portion of address is of arbitrary length
• Address format a.b.c.d/x
• x is number of bits in network portion of address

4
Allocating Addresses

• Assume abundant addresses are available starting
  at 194.24.0.0.
• Cambridge university needs 2048 addresses, it is
  given 194.24.0.0 to 194.24.7.255. Mask
  255.255.248.0.
• Oxford need 4096 addresses. Because the
  requirement is that must be on the boundary, it
  is given 194.24.16.0 to 194.24.31.255. Mask
  255.255.240.0.
• Edinburg needs 1024 addresses, is given
  194.24.8.0 to 194.24.11.255. Mask 255.255.252.0.

5
CIDR

• A router keeps routing table with entries
• IP address, 32-bit mask, outgoing line
• When an IP packet arrives, the router checks its
  routing table to find the longest match.
• Match means anding the IP address with the
  network address mask (111110000), and check if
  the result is the same as the network address.

6
CIDR

• Example.
• Cambridge 194.24.0.0/21 194.24.0.0 --
  194.24.7.255
• Edinburgh 194.24.8.0/22 194.24.8.0 --
  194.24.11.255
• (Available) 194.24.12.0/22 194.24.12.0 --
  194.24.15.255
• Oxford 194.24.16.0/20 194.24.16.0
  -- 194.24.31.255
• When a packet addressing to 194.24.17.4 arrives,
  where should it be sent to?
• And with all masks, find one that matches the
  longest.

7
CIDR Entry aggregation

• How does a router in Tallahassee route packet to
  C,E and O, assuming that he has only two outgoing
  links?
• All to New York.
• Can it reduce the size of his routing table?

C
E
N
O
H
T
8
CIDR Entry Aggregation

• From 194.24.0.0 to 194.24.31.255, all to N.
• So aggregate the three entries into one
  194.24.0.0/19.
• The N router can do the same thing.

C
E
N
O
H
T
9
CIDR

• If later the free address space 194.24.12.0/22
  194.24.12.0 -- 194.24.15.255 is assigned to
  Pittsburgh and has to go through Houston, what
  should the router at Tallahassee do?

C
E
N
P
O
H
T
10
CIDR

• When a packet arrives addressing 194.24.15.8, the
  router checks the routing table and there will be
  two matches 194.24.12.0/22 and 194.24.0.0/19.
  Pick the longest match.

11
NAT Network Address Translation

• IP address is a scarce resource.
• So, give a company only one or a few IP addresses
  used by the gateway router.
• Within the company, each machine has an unique IP
  address, chosen from
• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
• These addresses can only appear within a company
  but never on the outside Internet

12
NAT

• Whenever a machine wants to send a packet to the
  outside, the packet will be sent to the NAT box.
• The NAT box will convert the internal IP address
  to the real IP address of the company, and pass
  the packet to the gateway router.
• When there is a packet destined for an internal
  machine arrived at the router, what should the
  router and NAT box do?
• For IP packets carrying TCP or UDP, use port
  number. Other protocols are much more
  complicated.

13
NAT

• For IP packets carrying TCP or UDP, use port
  number.
• When an outgoing packet arrives at the NAT box,
• The IP address is replaced
• The source port number is replaced
• Header checksum is recomputed
• When a reply came for this process, use the
  replaced source port number as index to find the
  correct IP address and original port number.