The Bits and Bytes of Computer Networking 2

Question 1

How many bits is a MAC address and how are they represented?

Answer 1

48 bits, represented by 6 groupings of 2 hexadecimal digits.

Question 2

What is an octet, and how many hex digits can represent one?

Answer 2

Any number that can be represented by 8 bits. Can be represented by 2 hex digits.

Question 3

What are the first and last 3 octets of a MAC address?

Answer 3

First 3 = OUI - Organizationally Unique Identifier, assigned to individual hardware manufacturers by the IEEE.

Last 3 = assigned to devices by the manufacturer.

Question 4

What is a Unicast?

Answer 4

A one-to-one transmission from one point in the network to another point. Delivered from one sender to one specific recipient.

Question 5

What is a Multicast?

Answer 5

One-to-many, transmitted only to intended recipients in the network.

Question 6

What is a broadcast?

Answer 6

One-to-many, transmitted to all the hosts connected to the network. Sent using broadcast MAC address - all Fs.

Question 7

What is a Checksum and how is it calculated?

Answer 7

Calculated by performing a CRC - Cyclical Redundancy Check - against the contents of the packet. The receiving device will also perform a CRC to ensure received data is complete and uncorrupted.

Question 8

How many bits are IPv4 addresses and how are they represented?

Answer 8

32 bits, represented by 4 octets, described in decimal numbers, in dotted decimal notation.

Question 9

What is DHCP?

Answer 9

Dynamic Host Configuration Protocol, the process that automatically assigns an IP address to a new device from the network it has connected to.

Question 10

What is the difference between a dynamic and a static IP address.

Answer 10

A dynamic IP is assigned by DHCP, it can change each time a device connects to a network.

A static IP must be manually configured and does not change.

Question 11

What are data packets at the data link layer called?

Answer 11

Ethernet Frames.

Question 12

What are data packets at the network layer called?

Answer 12

IP Datagrams.

Question 13

What is the TTL field and what is it’s purpose?

Answer 13

Indicates the number of hops a packet can traverse before it is thrown away, each router that the packet reaches will decrement the field by one. The purpose is to prevent endless loops in the case of a misconfiguration.

Question 14

How are the octets in Classes A, B and C divided?

Answer 14

Class A - first octet = network ID.
last 3 octets = host ID.

Class B - first 2 octets = network ID.
last 2 octets = host ID.

Class C - first 3 octets = network ID.
last octet = host ID.

Question 15

What are Classes D and E used for?

Answer 15

D = Multicasting.
E = Testing purposes.

Question 16

What is the main issue with the Class system?

Answer 16

Class A networks are huge and there is not many of them available, Class C has many available networks but very limited possible host addresses on each. Overall the system is very limiting.

Question 17

What is ARP?

Answer 17

Address Resolution Protocol, used to discover the MAC address of a node with a certain IP address.

Question 18

What is an ARP table and what devices store them?

Answer 18

A list of IP addresses and the associated MAC addresses, stored on most network connected devices.

Question 19

What does a device do if it doesn’t have an entry in it’s ARP table for an IP address it wants to send data to?

Answer 19

It will send a broadcast ARP Message, when the device with the specified IP address receives the message it will send an ARP Response containing it’s MAC address.

Question 20

What is subnetting?

Answer 20

Splitting a large network into many smaller subnetworks.

Question 21

What is a Gateway Router?

Answer 21

A router that sits on the edge of a network, and connects the network to other networks - an entry / exit point for the network.

Question 22

What is a subnet mask? How does it represent the various IDs?

Answer 22

Tells devices what is the network ID, Subnet ID and Host ID. When in binary form - network/ subnet ID are represented by 1s, and the host ID represented by 0s. The device can compare this to the IP address in binary and understand which is which.

Question 23

What is CIDR?

Answer 23

Classless inter-domain routing. Expands on the subnet concept by using subnet masks to demarcate networks.

Question 24

What is a demarcation point?

Answer 24

Where one network/ system ends and another begins.

Question 25

What is CIDR notation and how does it work?

Answer 25

Slash notation, a tidier way to tell the subnet mask.

eg /27 will tell you that the first 27 bits are the network/ subnet IDs, and the remaining bits are the host ID.

Question 26

What are the 4 main columns in a routing table?

Answer 26

Destination network, the next hop, total hops, interface.

Question 27

What are routing protocols for, and name the two main categories?

Answer 27

How routers can learn the best routes to send data.

Interior routing protocols, exterior routing protocols.

Question 28

What are the 2 subcategories of interior routing protocols? Describe each.

Answer 28

Distance-Vector Protocols - older, a router using this protocol will send its routing table to every router directly connected to it, routers don’t know about overall network, only it’s neighbours, slow to react to far away changes.

Link State Protocols - newer, every router will tell every other router about what networks / routers it is connected to, each node will construct a map of the network connectivity, then calculate the fastest route itself.

Question 29

Where are interior gateway protocols used?

Answer 29

Used by routers to share information within a single autonomous network.

Question 30

Where are exterior gateway protocols used?

Answer 30

Used to communicate data between routers representing the edge of an autonomous system. (Gateway routers)

Question 31

What is an autonomous network? Provide an example of one.

Answer 31

A collection of networks that fall under control of a single network operator, eg, the many routers of a national ISP.

Question 32

What does IANA stand for and what are they responsible for?

Answer 32

Internet Assigned Numbers Authority - allocate IP addresses.

Question 33

Name 2 distance-vector protocols, and describe what they do.

Answer 33

RIP - Routing Information Protocol, finds the fastest route by lowest hop count. Doesn’t consider if route is congested.

EIGRP - Enhanced Interior Gateway Routing Protocol, exchanges routing information with its neighbours, and uses this info to calculate the best route.

Question 34

Name the only exterior gateway protocol in use today, and describe what it does.

Answer 34

BGP - Border Gateway Protocol, used by gateway routers, to connect one autonomous system with others. Protects each system, and finds the best routes to take.

Question 35

Name the most common link state protocol and describe what it does.

Answer 35

OSPF - Open Shortest Path First, widely used, learns about every router and subnet in the network, all routers share this information with each other through Link State Advertisements. Calculates the best routes using this info.

Question 36

What is non routable address space, and what / why is it used?

Answer 36

A range of IP addresses set aside, anyone can use them within a network, and with NAT they can communicate outside of that network.