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Question 1

What is special about private Ipv4 addresses?

Answer 1

Cannot be routed over the internet, used within an organization or site to allow devices to communicate locally.

Question 2

How do you allow a device with a private Ipv4 address to access resources outside the local network?

Answer 2

The address must be translated into a public address. NAT

Question 3

What is NAT? Why do we have NAT?

Answer 3

NAT allows networks to use private Ipv4 addresses internally and translate them to public addresses when needed.
A device wants to communicate with a device outside of its network.
The packet gets forwarded to the border router which performs the NAT process .
Translating the internal private address of the device to a public, outside, routable address.

The primary use of NAT is to conserve public Ipv4 addresses. (running out)

Question 4

What is an Inside address?

Answer 4

The address of the device that is being translated by NAT.

Typically private Ipv4 address.

Question 5

What is an Outside address?

Answer 5

The address of the destination device.

Question 6

What is a local address? What does it mean that they can Inside or outside?

Answer 6

A local address is any address that appears on the inside portion of the network.

Inside: Address of the source as seen from inside the network.

Outside: The address of the destination as seen from the inside network.

Question 7

What is a Global address? What does it mean that it can be either inside or outside?

Answer 7

A global address is any address that appears on the outside portion of the network.

Inside: Address of source as seen from the outside network.

Outside: The address of the destination as seen from the outside network.

Question 8

What is static NAT? In what areas is it useful?

Answer 8

Static NAT uses a one-to-one mapping of local and global addresses configured by the network admin that remain constant.

Useful for web servers or devices that must have a consistent address and devices that must be accessible by authorized personnel when offsite, but not my general public.

Question 8

What is Dynamic NAT? Where is it useful?

Answer 8

Dynamic NAT uses a pool of public addresses and assigns them on a first-come, first-served basis.

When an inside device requests access to an outside network, dynamic NAT assigns an available public Ipv4 address from the pool.

The other addresses in the pool are still available for use.

Question 9

What is Port Address Translation (NAT) NAT overload?

Answer 9

Maps multiple private IPv4 addresses to a single public Ipv4 address or a few addresses.

When the NAT router receives a packet from the client, it uses the source port number to uniquely identify the specific NAT translation.

PAT ensures that devices use a different TCP port number for each session with a server on the internet.

Question 10

What is special about PAT and original source ports?

Answer 10

PAT tries to preserver the original source port. If it is already used, PAT assigns the first available port number.
When there are no more ports available and there is more than one external address in the address pool, PAT moves to the next address to try to allocate the original source port.

Process continues until there are no more available ports or external Ipv4 addresses in the address pool.

Question 11

Benefits with NAT

Answer 11

Increases flexibility
Provides consistency
Allows the private Ipv4 address scheme to remain while allowing for easy change to a new public addressing scheme
NAT hides the Ipv4 addresses of users and other devices.
Conservers the legally registered addressing scheme by allowing privatization of intranets.
Conservers addresses through application port-lever multiplexing.

Question 12

Drawbacks with NAT

Answer 12

Increases forwarding delays(translation)
End to end addressing is lost
End to end Ipv4 traceability is lost
NAT complicates the use of tunneling protocols IPsec
Services that require the initiation of TCP connections from the otuside network, or stateless protocols, such as those using UDP can be disrupted.

Question 13

What are the two router functions?

Answer 13

Determine the best path using a router table(prefix(net address), prefix length)
Packet forwarding

Question 14

How can routes be considered a match? And what is the longest match?

Answer 14

There must be at least the number of matching bits indicated by the subnet mask of the route.

Longest match is a route in the table with the greatest number of far-left matching bits with packet’s dst IP address.

Question 15

What is directly connected nets?

Answer 15

Nets added to the table when a local interface is assigned Ip address/netmask and is active(up)

Question 16

Remote nets?

Answer 16

Routers learn about remote nets in two ways:

Static routes: Manually added to the routing table

Dynamic routing protocols: Routing protocol dynamically learns about remote nets

Question 17

How does packet forwarding work?

Answer 17

Data link frame with encap IP packet on ingress interface.

Router examines dst Ip address in pakcet header and checks routing table

Router finds the longest matching prefix in routing table

Router re-encapsulates the packet in data link frame and forwards to the egress interface, ARP or ICMPv6 ND is used to determine MAC address.

If no match, and no default route, packet is dropped.

Question 18

What is an IP routing table?

Answer 18

List of routes to the known networks derived from

Directly connected nets¨
Static routes
Dynamic routing protocols

Question 19

What is process switching?

Answer 19

An old Packet forwarding mechanism.

Forwards arriving pkt to control plane where CPU matches the dst addr to an entry in RT, determines the egress interface and forwards the pkt.

Question 20

What is fast switching?

Answer 20

Packet forwarding method. Used fast-switching cache to store next-hop info. CPU in control plane searches for match in the cache. If no match, uses Process swtiching. THe information flow then is stored in the cache. Another pkt with same dst will use the cache info without CPU intervention.

Question 21

What is Cisco Express Forwarding (CEF)

Answer 21

Most recent Cisco IOS packet forwarding mechanism. Builds a forwarding information base FIB and an adjacenct table. Table entries are change triggered instead of packet triggered (in fast switching) when something changes in the topology. When network has converged, FIB and adjacency table contain all info that a router needs.

Question 22

RT principles

Answer 22

a) Every router makes itw own decisions (based on RT info)
b) Info in RT of one touret is not necessarily identical to another router’s
c) Path routing info doesn’t provide “return” routing info.

Question 23

What is route source?

Answer 23

How route was learnt

Question 24

What is Dst net(prefix and length)

Answer 24

Remote net addr: prefix length identifies the min number of far-left bits that must match between pkt Ip addr and dst net for this route to be used.

Question 25

Administrative distance?

Answer 25

Lower values indicate preferred route source, reliability of router proto

Question 26

Metric?

Answer 26

Values assigned to reach the remote net. Preffered lower values

Question 27

Next-hop

Answer 27

Ip address of next hop(router)

Question 28

Router timestamps?

Answer 28

Time since route was learnt

Question 29

Exit interface

Answer 29

Egress interface to reach the destination

Question 30

Dynamic routing protocols, what do they do?

Answer 30

Automatically share info about reachability and status of remote nets. Perform several functions including network discovery and maintaining routing tables.

Question 31

Static routing

Answer 31

Used for smaller nets with only one path to an outside net; they provide security in larger nets for certain traffic types or links to other nets that need more control.

Question 32

Dynamic routing

Answer 32

Protos in any type of nets with more than a few routers! Scalable and adaptable to the change in topology (by finding better routes)

Question 33

IGP

Answer 33

Used for exchanging routing info withing a routing domain administrated by a single organization (autonomous system)

Question 34

EGP

Answer 34

Used for inter-domain routing, only one protocol (BGP) used for exchanging routing info between different organization (ASes) used by ISP to route packets over the internet.

Question 35

What is a routing protocol?

Answer 35

Set of processes algos, and messages, used to exchange routing info and populate routing table with the best path.

Discovery of remote nets

Maintaining up-to-date routing info

Choosing the best path to destination nets

Ability to find new best path when current path is no longer available.

Question 36

What are main components of routing protocol?

Answer 36

Data structures: Tables and databases kept in RAM

Routing Protocols and messages: Various kinds of messages to discover neighbour routers, exchange routing info, and other tasks to learn about network

Algorithms : Finite list of steps to determine the best path/route

Question 37

What is best path?

Answer 37

Selected based on metric to determine distance : best path=lowest metric different metrics used by each routing protocol.

Question 38

Load balancing

Answer 38

When there are two or more paths with equal costs metrics, both path can be used equally (equal cost lod balancing) Improves performance. ELB implemented automaitcally by dynamic routing protos; also enabled with static routes when multiple routes to the same net with different next-hop