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

2. Today CIDR Subnetting Private IP addresses
ICMP, IMAP, and DHCP Protocols

3. Packet Encapsulation
** Creative Commons:

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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?

11. 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

12. 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?

13. 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?

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

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

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

17. 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!
&

18. 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.

19. The University of Adelaide, School of Computer Science
21 February 2019
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

20. Routing with CIDR
Let’s look at an example from homework 5…

21. Special IP Addresses Network address: host id = all 0’s
Local broadcast address: all 1’s
Used during system startup
Directed broadcast address: host id = all 1’s
Routers will forward this broadcast address
Local host address (this computer): all 0’s
Loopback address
network id = 127, any host id (e.g )

22. Private IP Addresses Some addresses are not globally routable
IP packets created by these addresses cannot be transmitted into the public domain
Commonly used for home, office, and enterprise LANS

23. Private IP Addresses Address Range CIDR Number of Addresses– /8
16,777,216 –
/12
1,048,576 –
/16
65,535

24. Private IP addresses
Router uses Network Address Translation (NAT) to send IP packets from private IP addresses onto public networks
Router places it’s own IP address as destination
Maintains table, knows which host to route addresses
Router keeps translation table

25. 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

26. The University of Adelaide, School of Computer Science
21 February 2019
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

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

28. The University of Adelaide, School of Computer Science
21 February 2019
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

29. 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

30. Internet Control Message Protocol (ICMP)
The University of Adelaide, School of Computer Science
21 February 2019
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