AQA A Level Computer Science — Dynamic Host Configuration Protocol
DHCP stands for Dynamic Host Configuration Protocol. It is an application layer protocol used to automatically configure devices joining an IP network.
When a new device joins a network it needs several pieces of information before it can communicate:
DHCP automates this entire process, removing the need for an administrator to manually configure every device. A DHCP server maintains a pool of available IP addresses and assigns them dynamically to clients that request them.
Without DHCP, every device joining the network would need its IP address and network settings configured individually by an administrator. On a large network with hundreds of devices — many of which connect and disconnect frequently — manual configuration would be impractical and error-prone.
DHCP also prevents duplicate IP address conflicts, since the server tracks all allocated addresses and ensures each is issued only once at a time.
Fill in the missing words. Answers are not case-sensitive.
1. DHCP stands for Host Configuration Protocol.
The 'D' refers to the fact that addresses are assigned automatically rather than fixed.
2. DHCP operates at the layer of the TCP/IP model.
The same layer as HTTP, FTP and DNS.
3. As well as an IP address, a DHCP server also provides the and the .
One defines the network boundary; the other is the address of the router to reach external networks.
4. The DHCP server manages a of available IP addresses, from which it allocates addresses to clients.
A collective noun for a reserved set of resources ready to be assigned.
5. Without DHCP, a network administrator would need to configure the IP address of each device.
The opposite of automatic.
6. DHCP prevents IP addresses from occurring on the network, since the server tracks all allocations.
What happens when two devices accidentally share the same address?
Click Next Step to step through DHCP configuration one message at a time, or use Auto Play to watch the full sequence.
A new device has connected to the network. It has no IP address yet. Click Next Step to begin the DHCP negotiation.
The new client device broadcasts a
DHCPDISCOVER message across the network.
Because the client has no IP address yet, it uses source address
0.0.0.0 and sends to the broadcast address
255.255.255.255, so every device on the subnet receives
the message.
The message is asking: "Is there a DHCP server on this network? I need an IP address."
Any DHCP server that receives the DHCPDISCOVER responds with a DHCPOFFER. The offer includes:
If multiple DHCP servers exist on the network, the client may receive several offers. It will typically accept the first one that arrives.
The client sends a DHCPREQUEST message — again as a broadcast — to indicate which server's offer it is accepting. Broadcasting this message serves two purposes:
The DHCPREQUEST includes the IP address of the chosen server so that all servers can identify whether their offer was accepted.
The chosen DHCP server sends a DHCPACK (acknowledgement) back to the client, confirming that the IP address is now formally assigned. The client may now use all the configuration information provided.
The DHCPACK also confirms the lease period. Before the lease expires the device must either renew it or request a new address, ensuring the IP pool does not become exhausted by stale allocations.
Fill in the missing words. Answers are not case-sensitive.
1. The four stages of DHCP address negotiation are known by the acronym .
Discover, Offer, Request, Acknowledge — take the first letter of each.
2. In the Discover step the client uses the source IP address because it has not yet been assigned an IP address.
All zeros — a special address meaning "unknown/unassigned".
3. The DHCPDISCOVER message is sent to the
address
255.255.255.255 so that all devices on the subnet
receive it.
A message addressed to every device on the network simultaneously.
4. As well as an IP address, the DHCPOFFER includes the — the length of time for which the offered IP address may be used.
Think of renting something for a fixed period of time.
5. The DHCPREQUEST is sent as a so that any other DHCP servers that made an offer know their offer was not accepted and can release the reserved IP.
The same type of message as DHCPDISCOVER — not directed at a single host.
6. The final message in the DORA sequence is the (DHCPACK), confirming the IP address assignment to the client.
The 'A' in DORA — a confirmation that an action has been completed.
7. Both DHCPDISCOVER and DHCPREQUEST are messages because the client does not yet have a confirmed IP address it can use for direct communication.
They are sent to every device on the network, not a specific host.
8. The full name of DHCPACK is DHCP .
It is a confirmation — a reply that says "received and confirmed".
A DHCP server can operate in three different allocation modes, which determine how IP addresses from the pool are assigned to client devices:
The network administrator pre-configures the DHCP server by creating a mapping between each device's MAC address and a specific IP address.
When a device sends a DHCPDISCOVER, the server checks its MAC address and always returns the same pre-assigned IP. The address does not come from the general pool.
Also called: static allocation or DHCP reservation.
The DHCP server permanently assigns an IP address from its pool to a device the first time it connects. The assignment is remembered, so the same device always gets the same IP address on future connections.
Unlike manual allocation, no pre-configuration is required. Unlike dynamic allocation, the address is never returned to the pool for reuse.
The DHCP server assigns an IP address from its pool for a limited period — the lease period. When the lease expires, the device must request a new IP, which may be a different address. The old address is returned to the pool for reuse.
This is the most common form of DHCP, used on most home and business networks.
| Feature | Manual | Automatic | Dynamic |
|---|---|---|---|
| How IP is assigned | Pre-configured by admin (MAC → IP) | Automatically on first connect | Automatically from pool |
| Same IP every time? | ✓ Yes | ✓ Yes (after first use) | ✗ May change |
| Address returned to pool? | ✗ No | ✗ No | ✓ Yes (on expiry) |
| Lease period used? | N/A | N/A | ✓ Yes |
| Admin effort | High (per device) | Low | Low |
| Scalability | Low | Medium | High |
| Best used for | Servers, printers, fixed devices | Small, stable networks | Home/business LANs |
Fill in the missing words. Answers are not case-sensitive.
1. In allocation the administrator pre-configures a specific IP address for each device's MAC address before any connection is made.
2. In allocation the server permanently assigns an IP from its pool the first time a device connects, and the same address is given on every subsequent connection.
3. In allocation the server assigns an IP for a limited period known as the .
4. In manual allocation, each device is uniquely identified to the DHCP server by its address.
Media Access Control — a hardware address burned into the network interface card.
5. The most common type of DHCP allocation, used in most home and business networks, is allocation.
6. In dynamic allocation, when the expires the device must request a new IP address, which may differ from the previous one.
The time period for which a dynamically assigned IP address is valid.
7. An advantage of dynamic allocation over automatic allocation is that IP addresses are to the pool when they are no longer needed, making efficient use of the available address space.
The addresses go back to where they came from.
8. Manual allocation is also sometimes called allocation because the IP address does not change.
The opposite of dynamic or changing.