1. Computer Networks Fall, 2007 Prof Peterson
Welcome to CIS 235
Computer Networks
Fall, 2007
Prof Peterson

2. Final Preliminaries
Before we jump into chapter 1 we’ll cover two more pieces of background:
Binary numbers: can you convert between packed and unpacked representations?
Binary to decimal, CIDR
State machines – what are these?
Homework #2 will cover these – I’ll have it up later today.

3. Binary Numbers
We will write binary numbers in hexadecimal notation – each digit is 4 bits 0 = = = = = = = = = = 1001 A = 1010 B = 1011 C = 1100 D = 1101 E = 1110 F = 1111 What is 4FC in binary? What is in hex?

4. Binary to Base 10 Small numbers: Know these powers of 2:
Add up the values corresponding to 1’s: =
= 147

5. Big Numbers
We’ll stay approximate here – no need to worry about being exact. 210 = 1000 (1024) Examples: There are 28 class A networks, 216 class B networks, and 224 class C networks. Convert these to decimal!

6. Example When we go digging inside a packet, this is what we see.
We need to be able to break this down. There’s more structure
inside some of these fields. Let’s do an example. Make sure you
can break / assemble fields and convert hex to decimal.

7. IPv4 Address in Decimal
IP addresses are traditionally represented by taking the 32 bit IP address and dividing it into 4 8 bit sections. Each 8-bit value is converted to decimal. IP in Hex: F022A234 Convert these to decimal: F0, 22, A2, , 34, 162, 52 Add dots:

8. CIDR Notation
Classless Inter-Domain Routing CIDR notation is pervasive in networks. This consists of an IP address followed by a / and a number between 1 and 32. This number defines the number of bits to “keep” in the address. This really stands for a set of IP addresses. /8 = 8 high bits known, 24 bits remain /16 = 16 high bits known, 16 bits remain

9. CIDR examples
/32 means the single IP address /30 means IP addresses from tp Any part of the IP address that is below the “mask” should be /8 isn’t meaningful. Check out

10. State Machines
A state machine is a restricted computational process. It is allowed to only remember one thing: the current state (a finite number). At each state, the machine observes an input and makes a transition to a new state. We write states in circles, transitions with arcs between circles, and input values as labels on these arcs. Inside each circle is a state name. We’ll only deal with machines in which there is only one possible transition in each state.