1. Scaling the Network: Subnetting and Protocols
Networking
CS 3470, Section 1

2. Today
ARP, IMAP, and DHCP Protocols
Subnetting

3. Packet Encapsulation
** Creative Commons:

4. Address Resolution IP address is virtual
Not understood by underlying physical networks
IP packets need to be transmitted by the underlying physical network
Address resolution
Translating IP address to physical address
Address Resolution Protocol (ARP)

5. ARP A router has to know where to deliver packets on the local network
ARP is used to discover MAC addresses based on IP addresses
arp who-has tell node31.ceee.lab
arp reply is-at 00:60:08:ce:9d:3b
arp who-has node31.ceee.lab tell
arp reply node31.ceee.lab is-at 00:02:3f:b4:cd:87

6. ARP Cache Each computer maintains a cache table Exchanges ARP messages
IP address  hardware address mapping
Only about computers on the same network
Try out “/usr/sbin/arp –a” command
Exchanges ARP messages
To resolve IP addresses with unknown hardware addresses
Encapsulated in Ethernet data frame

7. ARP Protocol When a node sends an IP packet
To another node on the same physical network
Look up destination address in the ARP table
If not found
Broadcast a request to the local network
Whose IP address is this?
What info should the request message contain?

8. ARP Response The target node responds to sender (unicast?)
With its physical address
Adds the requester into its ARP table (why?)
On receiving the response
Requester updates its table
Other nodes upon receiving the request
Refresh the requester entry if already there
No action otherwise (why?)
Table entries deleted if not refreshed for a while
We can categorize the nodes in the local network into requester node, target node and all others. Lets see what each of them do in turn.
The target node responds only to the sender with its hardware address. Why not broadcast the reply also? All others may not necessarily be interested in communicating with target node and a broadcast incurs processing overhead at every node in the network. The target node adds the requester’s IP and hardware addresses into its ARP cache (if not already there). Why? It is likely that target node would also send IP packets to the requester node later and so it makes sense to avoid an ARP request broadcast by the target node that time.
The requester node on receiving the response, updates its ARP table.
All other nodes, receive only the request not the reply. They check if the requester has an entry in their caches. If found, they refresh that entry. Otherwise, no action taken. Why not add an entry for the requester? We don’t want to grow the ARP table unnecessarily.
Finally, a lifetime is associated with each entry in the ARP cache and an entry is deleted if it not refreshed within that time.

9. ARP Example
ARP broadcast by W requesting hardware address of Y

10. IP Address Configuration
May configure a network statically by giving each host it’s IP address and routing information (like gateway)
Or may configure a server to do this for you dynamically

11. The University of Adelaide, School of Computer Science
22 October 2017
DHCP Server
Dynamic Host Configuration Protocol (DHCP)
DHCP server is responsible for providing configuration information to hosts
There is at least one DHCP server for an administrative domain
DHCP server maintains a pool of available addresses
Chapter 2 — Instructions: Language of the Computer

12. DHCP Protocol State Protocol DHCPDISCOVER (client) DHCPOFFER (server)
DHCPREQUEST (client)
DHCPACK (server)
DHCPNAK (server)
DHCPINFORM (client)

13. The University of Adelaide, School of Computer Science
22 October 2017
DHCP
Newly booted or attached host sends DHCPDISCOVER message to a special IP address ( )
Rest of messages are unicast back and forth
Chapter 2 — Instructions: Language of the Computer

14. DHCP IP leases are valid for a predefined period of time (T1)
Leases are renewed at T1/2
Leases are released if they have not been renewed at the expiration of the lease time

15. Internet Control Message Protocol (ICMP)
The University of Adelaide, School of Computer Science
22 October 2017
Internet Control Message Protocol (ICMP)
Defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully
Destination host unreachable due to link /node failure
Reassembly process failed
TTL had reached 0 (so datagrams don't cycle forever)
IP header checksum failed
The ping application is a very common ICMP- message-generator
Chapter 2 — Instructions: Language of the Computer

16. IP Addressing
Classful addressing scheme separates groups of addresses into classes
Class A
8 bits used for network (256)
24 bits used for hosts and network devices (16,777,216)
Binary address starts with 0
Class B
16 bits for networks (65,536)
16 bits for hosts and network devices (65,536)
binary address starts with 10
Class C
24 bits for the network (16,777,216)
8 bits for the host (256)
Binary address starts with 110

17. Classless Inter-Domain Routing
Classful addressing scheme wasteful
IP address space exhaustion
Class B net allocated enough for 65K hosts
Even if only 2K hosts in that network
Solution: Classless Inter Domain Routing (CIDR)
Eliminate class distinction
No A,B,C
Keep multicast class D

18. Classless Addressing Addresses allocated in contiguous blocks
Number of addresses assigned always power of 2
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
This example is half of a class B!
network
part
host
/17

19. Subnet Motivation This network can have 215 = 32,768 hosts!
network
part
host
/17
This network can have 215 = 32,768 hosts!
Imagine the size of the routing tables if we had a flat network of all these hosts!
We want to split this network up into smaller networks

20. Subnet Motivation
network
part
host
/17
We probably want to split this network up into smaller networks (subnets) due to
Security reasons
Logistical reasons
Routing reasons

21. Let’s play with a small example
network
part
host
/24
Suppose you have this private class C network, and you need to divide it evenly
You will have hosts 0-127
Friend will have hosts

22. Let’s play with a small example
network
part
host
/24
Dividing the network into subnets involves using some of the host bits as the subnet ID
What bit of the host part of the address do we have to flip to signify >= 128 for the host ID?

23. Let’s play with a small example
network
part
host
/25
subnet ID
Can address hosts 0-127
Can address hosts
network
part
host
/25
subnet ID

24. Let’s play with a small example
network
part
host
/25
subnet ID
Now, how can routers easily figure out where destination IP address should be routed?
/25 or /25 subnet?

25. Let’s play with a small example
network
part
host
/25
subnet ID
A subnet number is the network part + subnet ID + zeros for the host
A subnet mask consists of all 1’s for the network+subnet ID and all 0’s for the host part
What is this subnet mask?

26. Let’s play with a small example
network
part
host
/25
subnet ID
Subnet mask:

27. Let’s play with a small example
network
part
host
/25
subnet ID
Subnet mask:

28. Subnet Masks We can figure out where to route by noting that
dest subnet = subnet mask & dest IP addr

29. Subnet Masks dest subnet = subnet mask & dest IP addr
Let’s say destination IP is and lets & with subnet mask
We send packet to /25 network!
&

30. Longest-Prefix Match Suppose two network IDs exist:
1) /16
2) /24
Suppose you have destination IP of
Both subnet mask & IP of 1 and 2 will yield match – what to do?
Longest-prefix match – route to network with the most matching host bits.

31. The University of Adelaide, School of Computer Science
22 October 2017
Subnetting
Notes
Would use a default router if nothing matches
Not necessary for all ones in subnet mask to be contiguous
Can put multiple subnets on one physical network
Subnets not visible from the rest of the Internet
Chapter 2 — Instructions: Language of the Computer

32. Routing with CIDR
Destination IP is BA.DB.EE.F2