4.3 DHCP and DNS

Question 1

What are two key functions of DNS?

Answer 1

1. It resolves hostnames to IP addresses.
2. It stores mail server details for domains.

DNS translates domain names into IP addresses, facilitating web navigation, and it maintains information necessary for email routing.

Question 2

What protocol does DNS use for queries?

Answer 2

UDP on port 53 but switches to TCP for large queries.

DNS primarily uses UDP for fast, lightweight queries. If a response exceeds 512 bytes, it switches to TCP for reliability.

Question 3

What are the main types of DNS records?

Answer 3

1. A
2. AAAA
3. MX
4. CNAME
5. PTR

A – Maps a domain to an IPv4 address.

AAAA – Maps a domain to an IPv6 address.

MX – Identifies mail servers for a domain.

CNAME – Maps an alias to a canonical domain name.

PTR – Resolves an IP address to a domain name (reverse lookup).

Question 4

What command can be used to query a DNS server for domain resolution?

Answer 4

nslookup (Windows) or dig (Linux/macOS).

nslookup is commonly used in Windows environments, while dig provides more detailed DNS query analysis in Linux and macOS.

Question 5

What’s the difference between authoritative and recursive DNS servers?

Answer 5

• Authoritative DNS: Stores domain name information and provides final answers.
• Recursive DNS: Queries other DNS servers to resolve names.

Recursive resolvers act as intermediaries, retrieving DNS information from authoritative servers when requested by clients.

Question 6

What DNS record is used for reverse DNS lookups?

Answer 6

PTR

(Pointer Record)

A PTR record maps an IP address to a hostname, enabling reverse lookups often used in email verification and security measures.

Question 7

True or False:

A root DNS server directs queries to TLD servers (e.g., .com, .org).

Answer 7

True

Root DNS servers are the foundation of the DNS hierarchy, managing requests for top-level domains (TLD) and guiding queries to authoritative servers.

Question 8

What is the difference between a recursive and an iterative DNS query?

Answer 8

• Recursive query: The DNS server resolves the request completely before responding.
• Iterative query: The DNS server provides referrals to other servers for resolution.

In recursive queries, the client relies on a DNS server to complete the resolution process, while in iterative queries, the client may need to contact multiple DNS servers until it gets a final answer.

Question 9

What is the purpose of a DNS forwarder?

Answer 9

It forwards DNS queries to external servers.

A DNS forwarder reduces the load on internal DNS servers by offloading external queries to another server, improving efficiency and response time for domain resolution.

Question 10

What is DHCP’s primary function?

Answer 10

It automates IPv4 configuration.

DHCP (Dynamic Host Configuration Protocol) assigns IP addresses dynamically to hosts, reducing the need for manual configuration in a TCP/IP network.

Question 11

Fill in the blank:

Hosts use the address ______ to communicate when they have not yet received an IP address from DHCP.

Answer 11

0.0.0.0

The address 0.0.0.0 is utilized by hosts that lack an assigned IP address, allowing them to send DHCP Discover messages. In contrast, 255.255.255.255 is used for local broadcasts.

Question 12

List four essential pieces of information needed to set up a DHCP server.

Answer 12

1. Default router
2. DNS server addresses
3. Reserved/excluded addresses
4. Scope

Default router – Gateway for client network access.

DNS server addresses – Resolves domain names to IPs.

Reserved/excluded addresses – Prevents conflicts with static IPs.

Scope – Defines the range of assignable IP addresses.

Question 13

What three values does a device need from DHCP for network access?

Answer 13

1. Device’s IP address
2. Default gateway IP address
3. Subnet mask

For proper configuration, a host needs its own IP address, the default gateway, and the subnet mask. It does not require the DNS server’s domain name but rather its IP address.

Question 14

What are the four main DHCP message types in the leasing process?

Answer 14

1. Discover
2. Offer
3. Request
4. Acknowledgment

Discover – Client searches for a DHCP server.

Offer – Server proposes an IP lease.

Request – Client asks for the offered IP.

Acknowledgment – Server confirms and finalizes the lease.

Question 15

Fill in the blank:

The IP address ______ is used for local broadcast in DHCP communications.

Answer 15

255.255.255.255

This broadcast address allows a client to send DHCP requests before receiving an IP address.

Question 16

What happens when the DHCP process fails for a client?

Answer 16

The client self-assigns an APIPA address.

If DHCP fails, the client uses Automatic Private IP Addressing (APIPA) from the range 169.254.0.0/16, allowing local communication only.

Question 17

What command is used on routers to support DHCP for remote subnets?

Answer 17

ip helper-address server-ip

This command forwards DHCP messages from clients in one subnet to a DHCP server located in another subnet.

Question 18

What information does a DHCP server typically store for each subnet?

Answer 18

• Subnet ID and mask
• Reserved (excluded) addresses
• Default router(s)
• DNS IP address(es)

These settings ensure proper DHCP lease allocation within a subnet.

Question 19

True or False:

DHCP servers are usually hardware in secure data centers.

Answer 19

False

DHCP servers are usually software-based and run on general-purpose servers or network appliances.

Question 20

What is the purpose of the DHCP relay feature?

Answer 20

It forwards DHCP messages between subnets.

DHCP relay agents allow clients on remote subnets to communicate with a centralized DHCP server.

Question 21

Fill in the blank:

DHCP clients use ______ messages to discover a DHCP server.

Answer 21

broadcast

DHCP clients broadcast a DHCP Discover message to locate available DHCP servers.

Question 22

What command is required for a switch to act as a DHCP client?

Answer 22

ip address dhcp

Applying this command to an interface allows a switch to obtain an IP address dynamically from a DHCP server.

Question 23

True or False:

DHCP can operate in dynamic, automatic, and static allocation modes.

Answer 23

True

Dynamic allocation assigns IPs temporarily, automatic allocation permanently assigns them, and static allocation reserves specific IPs for devices.

Question 24

What command verifies the IP address leased by a switch via DHCP?

Answer 24

show dhcp lease

This command displays the IP address and lease information assigned to the switch.

Question 25

What is the **significance** of the default gateway for routers using DHCP?

Answer 25

It defines the **default route**. ## Footnote The default gateway IP is used to *route traffic* from the local subnet to external networks.

Question 26

# True or False: If a client **doesn't renew**, the DHCP server **reclaims** the address.

Answer 26

True ## Footnote Expired leases allow the server to **reassign** IP addresses to new clients.

Question 27

What **key information** is included in DHCP client settings on a host?

Answer 27

* IP address * Subnet mask * Default router * DNS server addresses ## Footnote These settings are assigned dynamically by a *DHCP server*.

Question 28

What is the typical **IP address range** for APIPA?

Answer 28

169.254.0.1 to 169.254.255.254. ## Footnote **APIPA** provides automatic addressing in the *absence of a DHCP server*.

Question 29

What happens when **DHCP is disabled** on a Windows host?

Answer 29

The host must be **manually configured** with a static IP. ## Footnote Without **DHCP**, users must manually set an IP, subnet mask, default gateway, and DNS.

Question 30

List **two conditions** for a router to be a DHCP relay.

Answer 30

1. It must be configured with ip helper-address. 1. The subnet must not have its own DHCP server. ## Footnote These conditions ensure *DHCP messages* are relayed **properly**.