# 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.

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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 1 st digit to indicate network, and 1000 hosts per network. with 100 networks, use 1 st 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

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