material on communications

1. Objectives
Understanding how a “heterogeneous” collection
of networks can be connected to create an
internetwork
Understanding how to use the hierarchy of the
IP address to make routing decisions and make
routing in an internet more scalable

2. Internet Protocol (IP)
Part of TCP/IP
Used by the Internet
Specifies interface with higher layer
e.g. TCP
Specifies protocol format and mechanisms

3. IP Services
Primitives
Functions to be performed
Form of primitive implementation dependent
⌧e.g. subroutine call
Send
⌧Request transmission of data unit
Deliver
⌧Used by IP to notify user of arrival of data unit
Parameters
Used to pass data and control info

4. Parameters (1)
Source address
Destination address
Protocol
Recipient e.g. TCP
Type of Service
Specify treatment of data unit during transmission
through networks
Identification
Source, destination address and user protocol
Uniquely identifies PDU
Needed for re-assembly and error reporting
Send only

5. Parameters (2)
Don’t fragment indicator
Can IP fragment data
If not, may not be possible to deliver
Send only
Time to live
Send only
Data length
Option data
User data

6. Type of Service
Precedence
8 levels
Reliability for damage and loss
Normal or high
Delay
Normal or low
Throughput
Normal or high

7. IP Protocol

8. IP Addresses
To achieve scalability:
Reduce amount of info stored in each node
Reduce info exchanged between nodes
Introduce a 2-level hierarchy with networks at
the top level and hosts at the lower level –
hierarchical aggregation
Routers can have forwarding tables that list only
a set of networks numbers rather than all nodes
in each network
IP: get IP datagram to the right physical
network. Problem?

9. IP Addresses - Class A
Start with binary 0
Range 1.x.x.x to 126.x.x.x
Host portion is 24 bits
All allocated

10. IP Addresses - Class B
Start 10
Range 128.x.x.x to 191.x.x.x
Second Octet also included in network address
214 = 16,384 class B addresses
All allocated

11. IP Addresses - Class C
Start 110
Range 192.x.x.x to 223.x.x.x
Second and third octets also part of network
address
221 = 2,097,152 addresses
Nearly all allocated
See IPv6

12. Internetworking Using IP (1)
How do we get a datagram to a particular host or router on our
network?
The physical interface h/w on the host/router only understands the
addressing scheme of that particular network (e.g., Ethernet flat
addressing)
IP address link-level address
Encapsulate IP datagram inside a frame w Ethernet address
Solution?
Encode a host’s physical address in the host part of the IP address
⌧Class A/B/C?
⌧Ethernet?
Network admin manually configures a table of mappings
ARP ARP cache/table
⌧If (no mapping is found in ARP cache)
Broadcast ARP query w target IP address

13. Internetworking Using IP (2)
IP makes routing in an internet (or in the
Internet) “SOMEWHAT” scalable
Each router needs to know about all the
networks connected to the internet
Problem: today’s Internet has tens of thousands
of networks
Problem: large campus with many internal
networks
consider a Class C network with only 2/3 hosts
consider a physical network that needs only 256/257
hosts

14. Internetworking Using IP (3)
Conclusion: assigning one network number per
physical network uses up the IP address space
much faster than we would like. Plus, the more
the IP net numbers, the larger the forwarding
tables
Solutions:
Subnetting (for address utilization)
Route propagation techniques (16.1)

15. Subnets and Subnet Masks (1)
Take a single IP net number and assign it more than
one physical network, or “SUBNET”
They will all look as a single network
Examples: large campus or corporation. Form the
outside, all a router needs to know to reach any subnet
is the IP net number
Each physical net is assigned a subnet number
The host portion of the IP address is partitioned into
subnet number and host number
Bit positions containing this EXTENDED net number are
indicated by means of the “SUBNET MASK”

16. Subnets and Subnet Masks (2)
What happens when a datagram arrives from the rest of
the internet?
The subnet MASK allows the host to send directly or
send to a router
Hosts need not employ a subnet MASK. (T/F)
Hosts need not make routing decisions. (T/F)
Different subnets, on the same internet, can have the
same subnet MASK
Forwarding table entries
Are not of the form <subnet #, nextHop>. Why not?
<subnet #, subnet MASK, nextHop>

17. Subnets and Subnet Masks (3)
Different subnets, on the same internet, can have
different subnet MASKs. Why?
255.255.255.192
255.255.255.224
Default subnet MASK for a given class of addresses is a
null mask. What does this mean?
[(Subnet #)*2k] + host # = host portion in IP portion
10*25 + 1 = 65 (host C on p.547)
1*25 + 25 = 57 (host B on p.547)
Try it for hosts A and D
For a Class C subnet MASK of 255.255.255.224, the first
___ rightmost/leftmost bits of the host portion of the IP
address must be IDENTICAL for any 2 hosts on the
same physical network

18. Subnets and Subnet Masks (4)
For a Class C subnet MASK of 255.255.255.72, ___ bits
of the host portion of the IP address must be
IDENTICAL for any 2 hosts on the same physical
network
For a Class C subnet MASK of 255.255.255.0,
the first ___ rightmost/leftmost bits of the host
portion of the IP address must be IDENTICAL
for any 2 hosts on the same physical network
For a Class C subnet MASK of 255.255.255.224,
how many different subnets can this
accommodate?
2number of 1s in host portion of IP-like subnet mask

19. Routing Using Subnets