N10. Addressing

Question 1

Describe the hierarchy of global address allocation

Answer 1

Regional Internet Registry (RIR)
National Internet Registry (NIR)
Local Internet Registry (LIR)

Global registries delegate prefixes to RIRs.
RIRs delegate prefixes to LIRs.
LIRs delegate prefixes to ISPs
(ISPs allocate addresses to users)

Question 2

What is DHCP?

Answer 2

Dynamic Host Configuration Protocol
Automatically assigned address ‘leasing’ from a pool of addresses
Global or private address pool
Uses LAN broadcast

Question 3

How would a client request an address from DHCP?

Answer 3

1. discover DHCP servers and find an address
2. some DHCP server will send back an offer request with address and lease time
3. client sends request for which offer to accept and server sends “ack”
4. if client no longer needs address it can release it back to server
5. if client does not release address, it expires after lease time anyway

Question 4

What is routing (performed by an IP router)?

Answer 4

• process of discovering paths through the network
• requires a routing protocol
• metric value (cost) is assigned to each route

Question 5

What is forwarding (performed by an IP router)?

Answer 5

• process of transmitting packets using discovered routing information

Question 6

What is forwarding (performed by an IP host)?

Answer 6

• discovering locally available route(s) for destinations

- making local forwarding decisions

Question 7

How does an IP router use routing and forwarding to transmit packets to the correct destinations?

Answer 7

Routing protocol finds paths to destinations
- destination is another network
- discovery messages: discover hops along path
- Router typically has multiple interfaces
- Destination is reachable on a particular interface
Router constructs a forwarding table:
- for given destination, based on routing protocol info, select which is best interface to transmit packet
(similar process for forwarding on a host)

Question 8

How does the IP router choose the best route for forwarding?

Answer 8

Longest prefix match
- apply network mask of routing entry to destination address in packet
- the longest mask that produces a match between the local forwarding entry and destination address is used.
(if no prefix matches use default route 0.0.0.0)
i.e try 24 bit mask, then 20, then 16 etc for sizes of mask available

Note: lower metric value is better than higher if there are two possible routes

Question 9

What does NAT stand for?

Answer 9

Network Address Translation

Question 10

What are the problems with using NAT?

Answer 10

Loose end-to-end model
- IP addresses different at end point of connections and flows
- some applications may need special handling if they rely on addresses for configuration
NAT makes end-to-end security harder
Other problems e.g. single point of failure - if NAT breaks, need a router that knows about NAT config to replace it
Becomes difficult to have a mix of NAT addresses and global addresses if some clients need access to the internet

Question 11

What is the difference between a routing table and a forwarding table?

Answer 11

Routing table - entries for all possible routes

Forwarding table - reduced subset of routing tables (picks a route and gives info about forwarding for that route)