DNS Request and Port 53

Questions and Answers

What is the primary function of a NAT device?

To swap out the source IP address of the client with a globally routable IP address

What is the primary purpose of the well-known port 53 in a DNS request?

To listen for incoming DNS requests on the DNS server

What happens to the source port number when a packet goes through a PAT device?

It is swapped out with a unique source port number

What is the source IP address in the Layer 3 header of the DNS request?

10.1.10.101 Signup and view all the answers

What is the purpose of a DNS server?

To listen for incoming requests on port 53 Signup and view all the answers

What happens to a packet at Layer 2 when it reaches a server?

It is de-encapsulated Signup and view all the answers

What is the destination Layer 2 address in the Layer 2 header of the DNS request?

The MAC address of the default gateway Signup and view all the answers

What is the purpose of a NAT/PAT device in the context of a DNS request?

To un-translate the source IP address and port number Signup and view all the answers

What is the primary function of a Layer 2 switch in the network?

To make forwarding decisions based on Layer 2 information Signup and view all the answers

What is the source IP address of the response from a DNS server?

The IP address of the DNS server Signup and view all the answers

What is the purpose of a router in the network?

To make routing decisions based on Layer 3 information Signup and view all the answers

What happens to the destination IP address of a packet when it reaches a NAT/PAT device?

It is un-translated to the original IP address of the client Signup and view all the answers

Why is it necessary to perform address translation in the network?

To allow private IP addresses to be routed on the internet Signup and view all the answers

What is the primary purpose of NAT or PAT in the network?

To translate private IP addresses to public IP addresses Signup and view all the answers

What is the purpose of a Layer 2 address in the context of a DNS request?

To identify the destination device on a local network Signup and view all the answers

What is the destination port number of a response from a DNS server?

The source port number of the client Signup and view all the answers

What is the destination IP address in the Layer 3 header of the DNS request?

8.8.8.8 Signup and view all the answers

What is the role of the NAT/PAT device in the context of a DNS request?

To translate the source IP address and port number of the client Signup and view all the answers

What happens when a router receives a packet with a private IP address as the source address?

The router drops the packet Signup and view all the answers

What is the primary function of Layer 3 in the TCP/IP protocol stack?

To provide routing and addressing of packets Signup and view all the answers

What is the purpose of capturing traffic in two places in the demonstration?

To compare pre-NAT and post-NAT traffic Signup and view all the answers

What tool is used to verify that DNS is working in the demonstration?

nslookup Signup and view all the answers

What is the source port used by the client for the DNS request in the demonstration?

44,442 Signup and view all the answers

What happens to the source address in the traffic after address translation?

It changes to a private IP address Signup and view all the answers

What is the responsibility of the PAT device in the demonstration?

To perform NAT and PAT Signup and view all the answers

What is the purpose of using the terminal in the demonstration?

To use the nslookup tool Signup and view all the answers

What is the destination port used for the DNS request in the demonstration?

53 Signup and view all the answers

What is the Layer 3 header information in the original DNS request packet?

Source IP: 10.1.10.101, Destination IP: 8.8.8.8 Signup and view all the answers

What is the behavior of the PAT device regarding the source port number?

It may modify the source port number if necessary Signup and view all the answers

What is the purpose of the capture on Gig 0/0?

To capture post-NAT traffic Signup and view all the answers

What is the primary function of the application layer?

To provide functionality or service to the user Signup and view all the answers

What is the purpose of the DNS request in the example scenario?

To resolve a name to an IP address Signup and view all the answers

What protocol is used by the DNS service in the example scenario?

UDP, User Datagram Protocol Signup and view all the answers

What is the purpose of the source UDP port in the example scenario?

To track the DNS request Signup and view all the answers

What is the significance of the port number 52,768 in the example scenario?

It is a currently unused port number on the computer Signup and view all the answers

What happens to the DNS request after the UDP header is added?

It is routed to the network layer for further processing Signup and view all the answers

What is the relationship between the application layer and the rest of the protocol stack?

The application layer relies on the rest of the protocol stack for functionality Signup and view all the answers

What is the primary goal of the computer in the example scenario?

To retrieve a webpage from a server Signup and view all the answers

What is the primary function of the device labeled as 'Service Provider to Internet' in the given scenario?

To perform port address translation Signup and view all the answers

What is the command used to display the address translation entries on the Cisco router?

show ip nat translations Signup and view all the answers

What happens to the temporary UDP sessions in the address translation device after pressing the Up arrow key and Enter?

They are cleared and disappear Signup and view all the answers

Why is it important for network engineers and technicians to understand application layer services?

To troubleshoot application issues Signup and view all the answers

What is the main reason for permitting or denying certain types of traffic through the network?

To ensure application functionality Signup and view all the answers

What is the purpose of the 'show ip nat translations' command in the context of the given scenario?

To verify the address translation process Signup and view all the answers

What is the significance of the application layer in the protocol suite?

It logically delivers application services Signup and view all the answers

What is the advantage of understanding specific application layer services?

It helps troubleshoot application issues Signup and view all the answers

Study Notes

Application Layer and DNS Request

The application layer provides the functionality or service that a computer or user is trying to access.
The application layer uses the services of the lower layers (transport, network, data link, and physical) to function.
In this example, the application layer service is DNS (Domain Name System), which is used to resolve a name to an IP address.
The DNS request is made using UDP (User Datagram Protocol) as the transport layer protocol.
The source UDP port is a high, currently unused number (e.g. 52,768), and the destination UDP port is 53, which is the well-known port for DNS requests.

DNS Request Process

The DNS request is sent from the computer to the DNS server (e.g. Google at 8.8.8.8).
The request is packaged in a UDP header, which includes the source and destination ports.
The packet is then encapsulated in an IP header, which includes the source and destination IP addresses.
The IP header is then encapsulated in a Layer 2 header, which includes the source and destination MAC addresses.
The packet is sent to the default gateway, which forwards it to the DNS server.

NAT/PAT and Address Translation

The DNS request is sent from a private IP address (e.g. 10.1.10.101) to the public IP address of the DNS server (e.g. 8.8.8.8).
To make the request routable on the internet, the source IP address must be translated to a public IP address using NAT (Network Address Translation) or PAT (Port Address Translation).
The NAT/PAT device swaps the source IP address with a publicly routable IP address and remembers the translation.
When the response comes back, the NAT/PAT device translates the destination IP address back to the original private IP address.

DNS Response

The DNS server responds to the request with the IP address of the requested domain (e.g. www.cbtnuggets.com).
The response is packaged in a UDP header, which includes the source and destination ports.
The packet is then encapsulated in an IP header, which includes the source and destination IP addresses.
The IP header is then encapsulated in a Layer 2 header, which includes the source and destination MAC addresses.
The packet is sent back to the client through the NAT/PAT device, which translates the destination IP address back to the original private IP address.

Importance of Application Layer Services

Application layer services, such as DNS, HTTP, and HTTPS, are essential for network communication.
Understanding these services is crucial for network engineers and technicians to troubleshoot and configure networks.
Different application layer services use different ports and protocols, which can affect network configuration and security.

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Description

Understanding how DNS requests work and the significance of UDP port 53. Learn why DNS servers listen on this port and how it enables them to respond to requests.