Computer networks

1. Lesson Introduction

Every device connected to an IP network needs appropriate network configuration to communicate effectively.

A computer may need:

• An IP address

• Subnet mask

• Default gateway

• DNS server

• Lease information

These settings can be entered manually, but doing this for hundreds or thousands of devices would be time-consuming and could result in configuration errors.

The Dynamic Host Configuration Protocol (DHCP) solves this problem by allowing network devices to obtain IP configuration automatically from a DHCP server.

DHCP operates using a client-server model. A DHCP client requests network configuration, while a DHCP server provides the required configuration information.

2. Learning Objectives

By the end of this lesson, the learner should be able to:

1. Define DHCP.

2. Explain why DHCP is used.

3. Identify DHCP clients and servers.

4. Explain how DHCP assigns IP addresses.

5. Explain the DHCP DORA process.

6. Describe DHCP Discover.

7. Describe DHCP Offer.

8. Describe DHCP Request.

9. Describe DHCP Acknowledgement.

10. Explain DHCP leases.

11. Explain DHCP address pools.

12. Identify common DHCP options.

13. Explain DHCP relay.

14. Explain DHCP reservation.

15. Compare DHCP with static IP addressing.

16. Configure basic DHCP settings.

17. Troubleshoot common DHCP problems.

18. Explain the security risks associated with unauthorised DHCP servers.

3. Meaning of DHCP

DHCP stands for Dynamic Host Configuration Protocol.

It is a network protocol used to automatically provide network configuration information to devices on an IP network.

The official DHCP specification describes DHCP as a framework for passing configuration information to hosts on TCP/IP networks.

Instead of manually configuring every computer, a network administrator can configure a DHCP server with a pool of available addresses.

When a device joins the network, it can request configuration from the DHCP server.

4. Why DHCP Is Important

Without DHCP, an administrator may need to manually configure every device.

For example:

Computer 1:

IP address: 192.168.1.10

Subnet mask: 255.255.255.0

Gateway: 192.168.1.1

DNS: 8.8.8.8

Computer 2:

IP address: 192.168.1.11

Subnet mask: 255.255.255.0

Gateway: 192.168.1.1

DNS: 8.8.8.8

Doing this for 200 computers would take considerable time.

DHCP automates much of this process.

5. DHCP Client

A DHCP client is a device that requests network configuration from a DHCP server.

Examples include:

• Desktop computer

• Laptop

• Smartphone

• Tablet

• Network printer

• IP camera

• IP phone

• Smart television

• Server, when configured as a DHCP client

For example, when a laptop connects to Wi-Fi, it can request an IP address from the network's DHCP server.

6. DHCP Server

A DHCP server provides network configuration information to DHCP clients.

The server maintains information about:

• Available IP addresses

• Assigned IP addresses

• Lease durations

• Subnet masks

• Default gateways

• DNS servers

• Other DHCP options

A DHCP server can be a dedicated server or a service running on a router, firewall, operating system or network appliance.

7. DHCP Address Pool

A DHCP server normally has a range of IP addresses that it can allocate.

For example:

Network: 192.168.10.0/24

DHCP pool:

192.168.10.50 – 192.168.10.200

The server can allocate addresses from this range to clients.

Addresses outside the DHCP pool can be reserved for devices requiring manually configured addresses.

8. DHCP Scope

A DHCP scope defines the range and configuration parameters available to clients on a particular network.

A scope can contain:

• IP address range

• Subnet mask

• Excluded addresses

• Default gateway

• DNS server

• Lease duration

• Other options

For example:

Network: 192.168.20.0/24

DHCP range: 192.168.20.50–192.168.20.200

Gateway: 192.168.20.1

DNS: 192.168.20.1

9. DHCP Lease

A DHCP server normally does not give an address to a client forever.

Instead, the address is assigned for a specified period called a lease.

For example:

Lease duration: 8 hours

The client can normally renew the lease before it expires.

This allows unused addresses to eventually become available for other devices.

DHCP's dynamic allocation mechanism allows addresses to be reused when they are no longer needed.

10. DHCP and Static IP Addressing

There are two common ways to configure IP addresses.

Static IP

An administrator manually assigns the IP configuration.

Example:

Server:

192.168.10.10

DHCP

The device automatically obtains configuration.

Example:

Laptop → DHCP → 192.168.10.57

11. Static vs DHCP

12. What Information Can DHCP Provide?

DHCP can provide information such as:

• IP address

• Subnet mask

• Default gateway

• DNS server

• Domain name

• Lease duration

• Network-specific configuration options

The DHCP protocol supports mechanisms for delivering configuration parameters as well as allocating network addresses.

13. The DHCP DORA Process

A common DHCP exchange is remembered using the acronym DORA:

D – Discover

O – Offer

R – Request

A – Acknowledgement

The process allows a DHCP client to discover a server, receive an offer, request an address and receive confirmation.

14. DHCP Discover

The client begins by sending a DHCPDISCOVER message.

The client is effectively asking:

"Is there a DHCP server available?"

At this stage, the client may not yet have a usable IP address.

The Discover message is commonly broadcast on the local network.

15. DHCP Offer

A DHCP server receives the Discover message and may respond with a DHCPOFFER.

The offer can contain:

• Proposed IP address

• Subnet mask

• Lease duration

• Default gateway

• DNS information

For example:

DHCP Server:

"I can offer you 192.168.10.55 for this lease period."

Cisco's DHCP documentation describes the client sending DHCPDISCOVER and a DHCP server responding with configuration information including an IP address and lease information.

16. DHCP Request

The client chooses an offer and sends a DHCPREQUEST message.

The request tells the DHCP server that the client wants to use the offered configuration.

If multiple DHCP servers responded, the client identifies the offer it selected.

17. DHCP Acknowledgement

The DHCP server responds with DHCPACK.

This confirms that the client can use the assigned configuration.

The client can then configure its network interface and communicate on the network.

18. DORA Summary

19. Simple DORA Example

Imagine a laptop joining a Wi-Fi network.

Step 1:

Laptop → "I need network configuration."

DHCPDISCOVER

Step 2:

DHCP Server → "I can give you 192.168.1.50."

DHCPOFFER

Step 3:

Laptop → "I want that address."

DHCPREQUEST

Step 4:

DHCP Server → "Confirmed. You may use it."

DHCPACK

The laptop can now use the assigned network configuration.

20. DHCP Ports

DHCP uses UDP.

The standard DHCP server port is UDP 67.

The DHCP client port is UDP 68.

Therefore:

DHCP Server → UDP 67

DHCP Client → UDP 68

21. DHCP and Broadcasts

DHCP clients may initially use broadcast communication because they may not yet know the DHCP server's IP address.

This allows DHCP servers on the local network to detect the request.

22. DHCP Server on Another Network

A DHCP server does not necessarily have to be physically located on the same subnet as the client.

Routers can use DHCP relay functionality to forward DHCP requests toward a DHCP server on another network.

This is important in larger organisations.

For example:

Student VLAN
     |
   Switch
     |
   Router
     |
 DHCP Relay
     |
 DHCP Server

The DHCP server can serve clients across multiple subnets.

23. DHCP Relay

DHCP relay allows DHCP requests to cross a router.

This is useful when one central DHCP server serves several networks.

For example:

VLAN 10 → DHCP Server

VLAN 20 → DHCP Server

VLAN 30 → DHCP Server

Instead of installing a separate DHCP server for every VLAN, a router or Layer 3 switch can relay DHCP requests to a central server.

24. DHCP Reservations

A DHCP reservation allows a particular device to consistently receive a specific IP address.

The reservation is commonly associated with the device's MAC address or another client identifier.

Example:

Printer MAC Address → 00:11:22:33:44:55

Reserved IP → 192.168.10.20

Whenever the printer requests DHCP configuration, the server can provide the reserved address.

25. Why Use DHCP Reservations?

Reservations can be useful for:

• Network printers

• CCTV cameras

• Network storage

• Access points

• Certain servers

• POS devices

• Other infrastructure devices

The administrator can maintain centralised DHCP management while ensuring important devices receive predictable addresses.

26. DHCP Exclusions

An administrator may exclude certain addresses from the DHCP pool.

For example:

Network: 192.168.10.0/24

DHCP pool: 192.168.10.50–192.168.10.200

Reserved/static range:

192.168.10.1–192.168.10.49

This prevents the DHCP server from automatically assigning addresses intended for infrastructure devices.

27. DHCP Lease Renewal

DHCP clients do not necessarily wait until a lease completely expires.

They normally attempt to renew their lease during the lease period.

If renewal succeeds, the lease can continue.

If the DHCP server is unavailable, the client's ability to continue using the old configuration depends on the lease state and protocol rules.

28. DHCP Release

A DHCP client can release an IP address when it no longer needs it.

For example, a device leaving a network may release its address.

The server can then make that address available for another client.

29. DHCP and DNS

DHCP and DNS have different responsibilities.

DHCP:

Provides network configuration.

DNS:

Translates names into IP addresses and performs other name-resolution functions.

For example:

DHCP → "Your DNS server is 192.168.1.1."

DNS → "www.example.com is associated with this IP address."

30. DHCP and the Default Gateway

DHCP can tell a client which default gateway it should use.

For example:

Client IP: 192.168.10.50

Subnet mask: 255.255.255.0

Gateway: 192.168.10.1

The client uses 192.168.10.1 when it needs to communicate with destinations outside its local subnet.

31. DHCP in a Home Network

A home router commonly performs several functions:

• Router

• Firewall

• DHCP server

• Wireless access point

• Sometimes DNS forwarding

When a smartphone joins the home Wi-Fi network, the router can provide an IP address automatically.

32. DHCP in a Computer Laboratory

A computer laboratory may contain 40 computers.

Instead of configuring every computer manually, the administrator can configure a DHCP server.

For example:

DHCP range:

192.168.50.20–192.168.50.100

Gateway:

192.168.50.1

DNS:

192.168.50.1

When computers start, they request network configuration automatically.

33. DHCP in a School

A school may have different network segments:

• Administration

• Teachers

• Students

• Guest Wi-Fi

• ICT laboratory

Each network can have its own DHCP scope.

Example:

Administration:

192.168.10.0/24

Teachers:

192.168.20.0/24

Students:

192.168.30.0/24

Guests:

192.168.40.0/24

This can be combined with VLANs and DHCP relay.

34. DHCP in a Bank or Business

An organisation with multiple branches can centrally manage DHCP services.

Each branch may have its own subnet and DHCP configuration.

DHCP helps reduce the amount of manual IP configuration required for employee computers and other client devices.

35. DHCP and VLANs

DHCP is frequently used with VLANs.

For example:

VLAN 10 – Administration

VLAN 20 – Students

VLAN 30 – Guests

Each VLAN can have its own subnet and DHCP scope.

Example:

VLAN 10:

192.168.10.0/24

VLAN 20:

192.168.20.0/24

VLAN 30:

192.168.30.0/24

36. DHCP and Routers

A router can:

• Act as a DHCP server

• Forward DHCP requests

• Provide the default gateway

• Connect multiple DHCP-enabled networks

Small networks often use the router itself as the DHCP server.

Larger organisations may use dedicated DHCP servers.

37. DHCP and IPv4

DHCP is commonly associated with IPv4 network configuration.

For IPv4, DHCP can provide:

• IPv4 address

• Subnet mask

• Default gateway

• DNS server

• Lease information

DHCPv6 provides corresponding functionality for IPv6 networks, with important differences in operation.

38. DHCP Security

DHCP is convenient, but it also presents security risks.

One important risk is an unauthorised DHCP server.

For example, an attacker could connect a rogue DHCP server to the network and provide incorrect:

• IP addresses

• Default gateways

• DNS servers

• Other network information

The DHCP specification itself discusses the risk of unauthorised DHCP servers providing false or disruptive information.

39. Rogue DHCP Server

A rogue DHCP server is an unauthorised DHCP server operating on a network.

For example:

Legitimate DHCP Server:

Gateway → 192.168.1.1

Rogue DHCP Server:

Gateway → Attacker's device

Clients receiving the rogue configuration may experience:

• Loss of Internet access

• Incorrect DNS resolution

• Traffic interception

• Network disruption

40. DHCP Snooping

Managed switches can support DHCP snooping.

DHCP snooping can help distinguish trusted DHCP server ports from untrusted ports.

For example:

Switch Port 24 → Trusted DHCP server

Ports 1–23 → Untrusted client ports

The switch can then help prevent unauthorised DHCP responses from client-facing ports.

41. Common DHCP Problems

Common problems include:

• DHCP server unavailable

• DHCP pool exhausted

• Incorrect VLAN configuration

• DHCP relay misconfiguration

• Network cable failure

• Wi-Fi connectivity problems

• Firewall blocking DHCP traffic

• Rogue DHCP server

• Incorrect DHCP scope

• Incorrect subnet mask

• Incorrect gateway

• Incorrect DNS settings

42. DHCP Pool Exhaustion

Suppose a DHCP server has only 20 available addresses.

If 20 clients are already using them, a new client may not receive an IP address.

The administrator may need to:

• Increase the address pool

• Reduce unnecessary lease duration

• Remove abandoned leases

• Investigate devices consuming addresses

• Create additional subnets

43. Troubleshooting DHCP

When a computer does not receive an IP address:

1. Check the network cable or Wi-Fi connection.

2. Check whether the network interface is enabled.

3. Check the IP configuration.

4. Check whether the DHCP server is operating.

5. Check the DHCP scope.

6. Check whether addresses remain available.

7. Check VLAN configuration.

8. Check DHCP relay configuration.

9. Check switch connectivity.

10. Check firewall rules.

11. Test another client.

12. Check DHCP server logs.

44. Useful Windows Commands

The following commands are useful when troubleshooting DHCP on Windows.

View IP configuration:

ipconfig

Display detailed configuration:

ipconfig /all

Release the current DHCP configuration:

ipconfig /release

Request a new DHCP configuration:

ipconfig /renew

Clear the DNS resolver cache:

ipconfig /flushdns

Learners should understand that ipconfig /release and ipconfig /renew are particularly useful for testing DHCP address acquisition.

45. Linux DHCP Troubleshooting

Linux systems can use commands such as:

ip addr

to inspect IP addresses and interfaces.

Depending on the Linux distribution and network management system, tools such as:

nmcli

can also be used to inspect network configuration.

46. Practical Activity 1: Observe DHCP

On a Windows computer:

1. Open Command Prompt.

2. Run:

ipconfig /all

3. Identify:

   • IPv4 address

   • Subnet mask

   • Default gateway

   • DHCP server

   • DNS servers

4. Record the information.

47. Practical Activity 2: Renew a DHCP Address

On a test computer:

1. Open Command Prompt.

2. Run:

ipconfig /release

3. Observe the IP configuration.

4. Run:

ipconfig /renew

5. Run:

ipconfig /all

6. Compare the configuration before and after renewal.

48. Practical Activity 3: Configure DHCP in a Network Simulator

Using Cisco Packet Tracer:

1. Add a router.

2. Add a switch.

3. Add three PCs.

4. Connect the PCs to the switch.

5. Configure DHCP on the router.

6. Set the PCs to obtain addresses automatically.

7. Verify the addresses.

8. Test communication between the PCs.

49. Practical Activity 4: DHCP Pool Design

Create a DHCP scope for:

Network:

192.168.100.0/24

Gateway:

192.168.100.1

DHCP range:

192.168.100.50–192.168.100.200

DNS:

192.168.100.1

Students should calculate:

• Number of available DHCP addresses

• Addresses excluded from the pool

• Network address

• Broadcast address

50. Practical Activity 5: DHCP Troubleshooting

Create a simulated situation where a client cannot obtain an IP address.

Learners should investigate:

• Physical connection

• DHCP server

• DHCP scope

• Available addresses

• VLAN

• DHCP relay

• Client configuration

Students should document the cause and solution.

51. Assessment Questions

1. What does DHCP stand for?

2. What is the purpose of DHCP?

3. What is a DHCP client?

4. What is a DHCP server?

5. What is a DHCP scope?

6. What is a DHCP address pool?

7. What is a DHCP lease?

8. What does DORA stand for?

9. What is DHCPDISCOVER?

10. What is DHCPOFFER?

11. What is DHCPREQUEST?

12. What is DHCPACK?

13. Which transport protocol does DHCP use?

14. What are the standard DHCP server and client ports?

15. What is DHCP reservation?

16. What is DHCP relay?

17. Why is DHCP useful in a large computer laboratory?

18. What is a rogue DHCP server?

19. What is DHCP snooping?

20. Give five common causes of DHCP failure.

52. Practical Assignment

Design a DHCP system for a computer training centre with:

• 50 student computers

• 5 instructor computers

• 5 administrative computers

• 3 printers

• 3 wireless access points

• 2 servers

Create:

1. IP addressing plan.

2. DHCP scopes.

3. Address pools.

4. Excluded addresses.

5. Default gateways.

6. DNS configuration.

7. DHCP reservations for printers.

8. VLAN structure.

9. DHCP relay design if required.

10. Security measures against rogue DHCP servers.

53. Key Terms

DHCP – Dynamic Host Configuration Protocol.

DHCP Client – Device requesting network configuration.

DHCP Server – Device or service providing DHCP configuration.

DHCP Scope – Defined range and configuration parameters for DHCP clients.

Address Pool – Collection of IP addresses available for allocation.

Lease – Period during which a client may use an assigned IP address.

DORA – Discover, Offer, Request and Acknowledgement.

DHCPDISCOVER – Message used by a client to locate DHCP servers.

DHCPOFFER – Message containing an offered configuration.

DHCPREQUEST – Message used by a client to request configuration.

DHCPACK – Message confirming the DHCP configuration.

DHCP Relay – Mechanism that forwards DHCP requests between networks.

Reservation – Configuration that associates a particular client with a specific address.

Rogue DHCP – Unauthorised DHCP server.

DHCP Snooping – Switch security mechanism used to help control DHCP traffic.

54. Lesson Summary

DHCP is an important network protocol that automates IP configuration.

It can provide clients with:

• IP addresses

• Subnet masks

• Default gateways

• DNS server information

• Lease information

• Other network configuration options

The common DHCP process is DORA:

Discover → Offer → Request → Acknowledgement

DHCP reduces manual configuration and makes it easier to manage large networks.

It can be used in:

• Homes

• Schools

• Computer laboratories

• Offices

• Businesses

• Banks

• Universities

• Data centres

• Multi-branch organisations

However, DHCP must be properly configured and secured because unauthorised DHCP servers can disrupt network operation.

55. Further Learning Through Senior ICT Academy

More learning will be delivered through the Senior ICT Academy Campus by either a physical instructor or an online instructor.

Learners can continue with:

• IPv4 addressing

• Subnetting

• VLANs

• Routing

• DNS

• Network security

• DHCP server configuration

• Cisco networking

• Windows Server administration

• Network troubleshooting

Campus:

https://campus.seniorictacademy.com/login

Senior ICT Academy:

https://seniorictacademy.com