Question

A ping fails when performed from router R1 to directly connected router R2. The network administrator then proceeds to issue the show cdp neighbors command. What is CDP protocol? Why would the network administrator issue this command if the ping failed between the two routers?

You have been hired at AABC Inc. to help with an important network project. The network administrator has given you the internal IP address of 192.168.10.0/24 that need to have access to the corporate server at the IP address 209.165.200.254 across a router R1 that running dynamic NAT. Please provide your answers to the following sub-questions:

1.         How many hosts can this internal IP addresses provide?

2.         Please identify what is NAT?

3.         How many types of NAT to your knowledge?

4.         What are the advantages and disadvantages of NAT?

5.         What will be the actions that are involved when an internal host with IP address 192.168.10.10 attempts to send a packet to and external server at the IP address 209.165.200.254 across a router R1 that running dynamic NAT. (10 marks

Answer 1

Cisco Discovery Protocol:- The Cisco Discovery Protocol (CDP) is a Cisco proprietary Layer 2 (Data Link Layer) network protocol. To share information about other directly connected Cisco devices, such as the operating system version and IP address.

Cisco Discovery Protocol (CDP) messages received from a neighbor Cisco device are not forwarded to any other devices by default. This means that CDP is passed only to directly connected Cisco devices. Each Cisco device (which supports CDP) stores the messages received from neighbor devices in a table that can be viewed using the show cdp neighbors command.

If the ping failed between the two routers, Reasons:-

• Network administrator suspects a virus because the ping command did not work.
• Network administrator wants to verify Layer 2 connectivity.
• Network administrator wants to verify the IP address configured on router R2.
• The network administrator wants to determine if connectivity can be established from a non-directly connected network.

The show cdp neighbors command can be used to prove that Layer 1 and Layer 2 connectivity exists between two Cisco devices. FOR EG-

If two devices have duplicate IP addresses, a ping between the devices will fail, however the output of show cdp neighbors will be successful. The show cdp neighbors details could be used to verify the IP address of the directly connected device in case the same IP address is assigned to the two routers.

1) Internal IP Address:- It is also referred to as Private IP address , the internal IP address is the address that is assigned by your local network router that often begins with 192.168.x.x. Both the IPv4 and the IPv6 specifications define private IP address ranges.

   First block as "24-bit block"(single class A network ) the second as "20-bit block"(set of 16 contiguous class B network), and the third as "16-bit" block(set of 256 contiguous class C network).

• 10.0.0.0 - 10.255.255.255 (10.0.0.0/8 prefix)
• 172.16.0.0 - 172.31.255.255 (172.16.0.0/12 prefix)
• 192.168.0.0 - 192.168.255.255 (192.168.0.0/16 prefix)

2)Network Address Translation(NAT) is the process where a network device, usually a firewall, assigns a public address to a computer or group of computers inside a private network. It allows a router to modify packets to allow for multiple devices to share a single public IP address.This avoid the need to assign a new address to every host.

3) There are 3 different types of NAT:-

• Static address translation (Static NAT): This type of NAT is used when a single inside address needs to be translated to a single outside address or vice versa.
• Dynamic address translation (Dynamic NAT): This type of NAT is used when an inside address (or addresses) need to be translated to an outside pool of addresses or vice versa.
• Overloading (Port Address Translation (PAT): This type of NAT is a variation on dynamic NAT. With dynamic NAT, there is always a one to one relationship between inside and outside addresses; if the outside address pool is ever exhausted, traffic from the next addresses requesting translation will be dropped. With overloading, instead of a one to one relationship, traffic is translated and given a specific outside port number to communicate with; in this situation, many internal hosts can be using the same outside address whil utilizing different port numbers.

4) ADVANTAGE:-

• It can prevent the depletion of IPv4 addresses.NAT save the addresses using application port-level multiplexing.
• If a network uses a public IP address, first the administrator will get an address space as a network grows, But in case of using private address and NAT for external traffic using some addresses, an organization does not have to purchase IP addresses for every computer in use there is a significant cost saving due to using the process of Network Address Translation.
• NAT allows to use your own private IPv4 addressing scheme and prevent the internal address changes in case the service provide is changed.

DISADVANTAGE:-

• It slows down the network performance.
• The end-to-end addressing is lost. Several Internet protocols and applications basis on end-to-end addressing from the source to the destination therefore many applications do not work with NAT.
• Consumes the processor and memory because NAT need to translate IPv4 addresses for all incoming and outgoing IPv4 datagram.

5) Actions that are involved when an internal host with IP address 192.168.10.10 attempts to send a packet to and external server at the IP address 209.165.200.254 across a router R1 that running dynamic NAT:-

a) The host sends packets that request a connection to the server at the address 209.165.200.254

b) R1 checks the NAT confguration to determine if this packet should be translated.

c) If there is no translation entry for this IP address, R1 determines that the source address 192.168.10.10 must be translated.

d) R1 selects an available global address from the dynamic address pool.

e) R1 replaces the address 192.168.10.10 with a translated inside global address.