Chapter 21 Exercises
1
A router forwards packets between
networks.
(Given a destination host address,
it must be able to figure out which
network that machine is on.)
2
Big Question
Addresses have to be assigned to hosts
based on the network they are on.
How can that be done?
3
Exercise
You are the CIO of a company and get 10000
addresses total (numbered 0 to 9999) and have 10
networks each managed by a different
administrator, and you want each network
administrator to be able to assign addresses
independently. How do you allocated addresses?
(Remember how routers work: move packets based
on network addresses, not host addresses.)
4
Answer these questions about your scheme:
• Does every host get a unique address?
• How many networks can be supported? How
many hosts per network?
• How does a router know, from a destination
address in a packet, which network the host
is on?
5
Exercise
What if you had 100 networks in your company?
What scheme would you use? Can you answer
the questions from the previous slide?
6
Exercise
What if you had 12 networks in your company?
What scheme would you use?
7
Your company is connected to the Internet via
one router, so other computers can
communicate with hosts in your network. What
do the routers in the Internet need to know to
send packets to your company’s networks?
8
Summary:
• with 10 networks, use 1st digit to indicate
network, and 1000 hosts per network.
• with 100 networks, use 1st 2 digits to indicate
network, and 100 hosts per network.
• (or some other rule…)
• can be decided by CIO/network admin.
9
BUT: somehow routers in your network have to
be configured to know which part of the address
is _________ _______ and ______ ________.
Come up with a scheme to indicate this.
10
Question: does a router need this
“indicator” to identify a host?
11
Now: switch to binary instead of decimal.
(Why does IP use binary addressing instead of
decimal?)
Suppose you have 1 byte IP addresses. How
many total IP addresses in the world?
Suppose you have 4 (sub) networks. How to
allocate addresses?
12
How many hosts per network?
Can you compute the network from the host IP
address?
What is the subnet mask?
What are the formulas for computing max hosts
supported on a subnet and max # of networks?
13
32-bit addresses
How many bytes is that?
How many total addresses (theoretically)?
How is a 32-bit address written?
What are common divisions between network
part and host part?
14
Classful Addressing
(p. 348)
Why was it invented?
How many class A networks? B? C? (What are the
“implied” masks for these networks?)
What if you needed 500 addresses? What class
address would you get? How many wasted
addresses?
15
Classless Inter-Domain Routing (CIDR)
What changed from classful addressing?
What must routers know now in order to
forward packets correctly?
16
What kind of address is this? (host? network?
limited bcast? directed bcast?)
128.211.0.16/28
10.1.0.77/8
128.211.0.17/28
128.211.0.31/28
255.255.255.255/32
17