Network Layer III

Question 1

DHCP: Dynamic Host Configuration Protocol

Answer 1

Dynamic IP address allocation: Host gets an IP for a limited time (called a lease).

Lease renewal: Host can renew the lease to keep the IP.

Address reuse: IPs are reused efficiently — if a device leaves, its IP can be assigned to another device.

Supports mobility: Devices can join and leave networks easily, getting new IPs as needed.

Question 2

DHCP: more than IP addresses

Answer 2

▪ address of first-hop router for client
▪ name and IP address of DNS sever
▪ network mask (indicating network versus host portion of address)

Question 3

NAT: network address translation

Answer 3

NAT: all devices in local network share just one IPv4 address as
far as outside world is concerned

▪ all devices in local network have 32-bit addresses in a “private” IP
address space (10/8, 172.16/12, 192.168/16 prefixes) that can only
be used in local network
▪ advantages:
▪ just one IP address needed from provider ISP for all devices
▪ can change addresses of host in local network without notifying
outside world
▪ can change ISP without changing addresses of devices in local
network
▪ security: devices inside local net not directly addressable, visible
by outside world

Question 4

NAT: network address translation

Answer 4

Outgoing packet Replace (source IP, port) → (router’s public IP, new port) and remember this mapping

Incoming packet Use mapping to replace (router’s public IP, port) → (original device IP, port)

Question 5

Making routing scalable

Answer 5

scale: billions of destinations:

▪ can’t store all destinations in
routing tables!

▪ routing table exchange would
swamp links!

That’s why hierarchal routing is important

Question 6

Internet approach to scalable routing

Answer 6

aggregate routers into regions known as “autonomous
systems” (AS) (a.k.a. “domains”)

intra-AS (aka “intra-domain”):
routing among routers within same
AS (“network”)
▪ all routers in AS must run same intra
domain protocol
▪ routers in different AS can run different
intra-domain routing protocols
▪ gateway router: at “edge” of its own
AS, has link(s) to router(s) in other
AS’es

inter-AS (aka “inter
domain”): routing among
AS’es
▪ gateways perform inter-domain
routing (as well as intra-domain
routing)

Question 7

Interconnected ASes

Answer 7

forwarding table configured by
intra- and inter-AS routing
algorithms

intra-AS routing determine entries for
destinations within AS

inter-AS & intra-AS determine entries
for external destinations

Question 8

Intra-AS routing: routing within an AS

Answer 8

RIP: Routing Information Protocol [RFC 1723]
* classic DV: DVs exchanged every 30 secs
* no longer widely used

▪EIGRP: Enhanced Interior Gateway Routing Protocol
* DV based
* formerly Cisco-proprietary for decades (became open in 2013 [RFC 7868])

▪ OSPF: Open Shortest Path First [RFC 2328]
* link-state routing
* IS-IS protocol (ISO standard, not RFC standard) essentially same as OSPF

Question 9

OSPF (Open Shortest Path First) routing

Answer 9

Open protocol
Publicly specified (RFC 2328), non-proprietary.

Link-state routing
* Routers measure cost of each directly connected link (e.g. bandwidth, delay).
* Flood Link-State Advertisements (LSAs) to all routers in the AS (over IP).

Global topology map
Every router builds the same complete network graph.

Shortest-path computation
Uses Dijkstra’s algorithm on that graph to populate its forwarding table.

Multiple cost metrics
You can weight routes by bandwidth, delay, or a combination.

Security
All OSPF messages are authenticated to prevent spoofing.

Question 10

Hierarchical OSPF

Answer 10

Designed for scalability in large networks.

Backbone Area (Area 0)
Central core that connects all other areas.
Only backbone routers operate here.

Local Areas (Area 1, Area 2, etc.)

Contain internal routers that only know about their area.