3.4 Given a scenario, implement IPv4 and IPv6 network services
Dynamic Addressing
📘CompTIA Network+ (N10-009)
Dynamic Addressing with DHCP
Dynamic addressing is a method for automatically assigning IP addresses to devices on a network. This is done using DHCP (Dynamic Host Configuration Protocol). DHCP helps network administrators manage IP addresses efficiently, avoiding conflicts and reducing manual work.
When a device connects to a network, it needs an IP address to communicate. DHCP automatically assigns this IP address from a pool of available addresses.
1. DHCP Scopes
- A scope is a range of IP addresses that a DHCP server can assign to devices.
- Think of it as a “pool” of addresses available for devices in a particular network.
- A scope can include:
- Start and end IP addresses (range)
- Subnet mask
- Default gateway
- DNS server information
Example in IT environment:
If you have a network 192.168.1.0/24, you might configure a scope from 192.168.1.100 to 192.168.1.200 for DHCP clients. Devices connecting to this network automatically get an IP in that range.
2. DHCP Reservations
- A reservation is when you reserve a specific IP address for a particular device based on its MAC address.
- This ensures that the device always gets the same IP address from the DHCP server, even though DHCP is usually dynamic.
Use case in IT:
A server or printer may need a fixed IP to ensure other devices can always reach it reliably. Instead of manually setting the IP, you reserve it on the DHCP server.
3. Lease Time
- Lease time is the amount of time a device can use an IP address assigned by DHCP before it must renew the lease.
- Once the lease expires, the DHCP server can reassign the IP to the same device or another device.
Key points for the exam:
- Short lease times: Useful for networks with many transient devices (like guest Wi-Fi).
- Long lease times: Useful for devices that stay connected for long periods (like servers or workstations).
IT example:
A laptop in an office might get a 24-hour lease. If it stays on the network for longer, it automatically renews the lease.
4. DHCP Options
- Options are extra configuration settings that DHCP can provide to clients besides IP addresses.
- Common DHCP options include:
- Default gateway
- DNS servers
- Domain name
- WINS servers
- NTP servers (time servers)
Why it’s useful:
Options allow network devices to automatically get all necessary network information without manual configuration.
Example in IT:
When a new workstation connects, it gets not only an IP address but also DNS server info, default gateway, and network time server address.
5. DHCP Relay / IP Helper
- DHCP Relay (sometimes called IP Helper) allows devices on a subnet to get an IP from a DHCP server located on a different subnet.
- Normally, DHCP messages are broadcast-based and cannot cross routers. DHCP relay forwards these messages across networks.
IT example:
If the DHCP server is on 192.168.10.0/24 and a client is on 192.168.20.0/24, the router can relay DHCP requests so the client still gets an IP address.
6. DHCP Exclusions
- Exclusions are IP addresses within a DHCP scope that should not be assigned to clients.
- These are used for devices that need a static IP manually assigned, like servers, routers, or printers.
Example in IT:
In a DHCP scope from 192.168.1.100 to 192.168.1.200:
- Exclude 192.168.1.101–192.168.1.110 for servers.
- The DHCP server will not assign these IPs to any client.
Summary Table for Quick Review
| Feature | Purpose / Explanation | IT Example |
|---|---|---|
| Scope | Range of IPs DHCP can assign | 192.168.1.100–192.168.1.200 |
| Reservation | Assign a specific IP to a device | Printer always gets 192.168.1.150 |
| Lease Time | Duration a client can use the IP | Laptop gets IP for 24 hours |
| Options | Extra network info provided by DHCP | DNS, gateway, domain, time server |
| Relay / IP Helper | Forward DHCP requests across subnets | Router forwards DHCP request from 192.168.20.0/24 to server on 192.168.10.0/24 |
| Exclusions | IPs within a scope that won’t be assigned | 192.168.1.101–192.168.1.110 reserved for servers |
Exam Tips
- Remember the difference between reservations and exclusions:
- Reservation = guaranteed IP for a device
- Exclusion = IPs DHCP won’t assign at all
- Understand lease time impact on network behavior.
- Know why DHCP relay / IP helper is required: DHCP broadcasts do not cross routers by default.
- Be able to identify common DHCP options that help clients connect and function on the network automatically.
