DHCP & DNS

Question 1

What does DHCP stand for?

Answer 1

Dynamic Host Configuration Protocol

Question 2

What does DHCP do?

Answer 2

It automatically allocates and IP address to a device that requests one from a given pool of IP addresses

Question 3

How many steps does DHCP handshake have?

Name them.

Answer 3

4 Steps.
Discover, Offer, Recquest, Acknowledge.
Also known as DORA handshake

Question 4

What is the source address when a device requests an IP address?

Answer 4

0.0.0.0

Question 5

What is the destination address when a device requests and IP address?

Answer 5

255.255.255.255

Question 6

What extra configuration is required if a DHCP server is not located on the local subnet?

Answer 6

Connecting router(s) need to be configured with ‘ip helper’ address that will forward DHCP Requests outside of the local subnet

Question 7

What layer 4 protocol does DHCP use and what ports does that protocol use?

Answer 7

UDP (User Datagram Protocol). Ports 67 and 68

Question 8

How to exclude a range of addresses from a DHCP pool?

Answer 8

use ‘ip dhcp excluded-address [first address in range] [last address in range]’ command

Question 9

What are the steps to configure a DHCP server on a Cisco router?
Provide commands as well.

Answer 9

1. Create range of excluded addresses - ‘ip dhcp excluded-address [first] [last]
2. Create a new DHCP pool - ‘ip dhcp pool [name]’
3. Provide the pool with range of ip addresses to offer - (while in dhcp-config) ‘network [subnet address] [subnet mask]
4. Define default router - (in dhcp-config) ‘default-router [ip address]’
5. Define DNS server - (in dhcp-config) ‘dns-server [ip address]’

Question 10

What command is used to get information of the DHCP pool(s) configured on a L3 device

Answer 10

show ip dhcp pool

Question 11

What is a DHCP binding?

Answer 11

It’s an entry in DHCP database that shows mapings between MAC addresses of devices asking for IP addresses and IP addresses given to them

Question 12

Is ‘ip helper-address’ a global command?

Answer 12

No. It’s an interface command

Question 13

Why is it important to make the dhcp server aware of vlans on the network?
How can this be done?

Answer 13

DHCP server needs to know where to send the DHCP messages when asked for IP address allocations.
Via static routes or dynamic routing protocols like RIP, EIGRP or OSPF.

Question 14

What does DNS stand for and what does it do?

Answer 14

DNS stands for Domain Name System.

It translates IP addresses used my machines to human readable names like ‘google.com’

Question 15

Can a device have more than one DNS server addresses configured?

Answer 15

Yes

Question 16

What is an ‘A record’ in DNS?

Answer 16

A record holding a Domain name

Question 17

What is AAAA record in DNS

Answer 17

That’s a record holding a domain name in IPv6

Question 18

Does DNS use TCP or UDP Layer 4 protocol?

Answer 18

It uses both

Question 19

What is the well known port number used by DNS?

Answer 19

53