The Internet Layer

Question 1

The Internet Layer provides protocols for:

Answer 1

1 addressing: How do we know where we are sending packets
2 forwarding: How do we move them on their way
3 routing: How to decide which route to take
4 reporting: How do you signal at the lowest level that things have gone wrong

Question 2

The Internet Protocol (IP) gives :

Answer 2

every host and router network interface an address

Question 3

The most important fields of an IP datagram header:

Answer 3

the source address and destination address

Question 4

what is the internet?

Answer 4

interconnection of different types of connections

Question 5

What is a router?

Answer 5

a router exchanges forwarding tables that determine the next best hop route

Question 6

What is a header in the network packet?

Answer 6

its the front

Question 7

An IP address is divided into :

Answer 7

a network prefix
a host suffix
written network / host

Question 8

What is the dotted decimal notation?

Answer 8

It is a way to write IP addresses: bytes are written as decimal numbers, separated by dots

Question 9

What private addressed may not be used on the public internet:

Answer 9

10. 0.0.0/8 — 10.255.255.255/8
11. 16.0.0/12 — 172.31.255.255/12
12. 168.0.0/16 — 192.168.255.255/16

Question 10

What does an IP router do?

Answer 10

when a packet is coming from a router it comes in a particular interface but it must go out in another and not the same one otherwise it would start looping.
so a router forwards packest on the appropriate interface.

stores a forwarding table which gives an interface for each possible destination address

periodically exchanges forwarding tables with its neighbours, and then recomputes its own.

Question 11

What does an IP router use?

Answer 11

An IP router uses longest prefix address matching, and
benefits from addresses being assigned in hierarchical
fashion

Question 12

What is ICMP?

Answer 12

Internet Control Messaging Protocol

Question 13

What does the ICMP do?

Answer 13

allows hosts and routers to communicate (often error) information to each other.

So the Internet Control Messaging Protocol is a low-level protocol that enables a router to signal back that a packet cannot be delivered. (It makes no attempt to recover it )

Question 14

ICMP Messages:

Answer 14

ICMP Type Code Description
0 0 echo reply
3 0 destination network unreachable
3 1 destination host unreachable
3 2 destination protocol unreachable
3 3 destination port unreachable
3 6 destination network unknown
3 7 destination host unknown
8 0 echo request

Question 15

What does a router do if it doesn’t know what port to forward a packet to?

Answer 15

Sends it to a default one