DHCP (IPv4 and IPv6) Flashcards
(24 cards)
Describe how a PC obtains an IPv4 address using DHCPv4.
- Discover – PC sends a broadcast to find DHCP servers.
- Offer – DHCP server replies with an available IP address.
- Request – PC requests to use the offered IP.
- Acknowledge – DHCP server confirms and leases the IP to the PC.
By default, where must the DHCP server for a PC be? How can we overcome this restriction?
By default, the DHCP server must be on the same local network (broadcast domain) as the PC, because DHCP Discover messages are broadcasts and don’t cross routers.
We overcome this by using a DHCP relay agent (often configured on a router) to forward DHCP requests to a server on another network.
Why might we exclude some addresses from the range of addresses in a DHCP pool?
To reserve IP addresses for static devices like servers, printers, or network equipment, preventing IP conflicts between static and dynamically assigned addresses.
When testing a system, why should you try applying a static IP address to a PC before you try to acquire an address using DHCP for it?
To verify basic network connectivity and ensure there are no issues with the PC’s network interface or cabling before troubleshooting DHCP problems.
What is Dynamic Host Configuration Protocol v4 (DHCPv4)?
Assigns IPv4 addresses and other network configuration information dynamically.
It leases addresses from a pool, instead of needing manual setup for each device.
Is an extremely useful and timesaving tool for network administrators – especially when there are hundreds or thousands of end-devices (PCs or servers) that need IP addresses.
Big networks usually use a dedicated DHCP server. But in small networks (like home or small offices), a Cisco router can act as a DHCP server too.
What is meant when DHCPv4 is said to work in a client-server mode?
When a client communicates with a DHCPv4 server, the server assigns or leases an IPv4 address (from a pool) to that client.
What is DHCPv4 address leasing?
A client connects to the network with that leased IPv4 address until the lease expires (typically anywhere from 24 hours to a week or more). The client must contact the DHCP server periodically to extend the lease.
This lease mechanism ensures that clients that move or power off do not keep addresses that they no longer need.
When a lease expires, the DHCP server returns the address to the pool where it can be reallocated as necessary.
What are the steps to renew a DHCPv4 lease?
- DHCP Request
- Before the lease runs out, the client sends a DHCPREQUEST directly to the server that gave it the IP address.
- If there’s no reply in time, the client broadcasts the request so other DHCP servers can respond.
- DHCP Acknowledgment
- The server replies with a DHCPACK, confirming the lease is renewed.
- If accepted, the client keeps using the IP address.
What are the steps to configure a DHCPv4 server?
- Exclude IPv4 addresses.
- Excluded addresses should be those addresses that are statically assigned to routers, servers, printers, and other devices that have been, or will be, manually configured.
- Define a DHCPv4 pool name.
-The ip dhcp pool pool-name command creates a pool with the specified name and puts the router in DHCPv4 configuration mode. - Configure the DHCPv4 pool.
-The address pool and default gateway router must be configured.- Can also define DNS server, domain name, and duration of the DHCP lease.
What is the purpose of a DHCPv4 relay?
To forward DHCP messages between clients and a DHCP server on a different network, since DHCP broadcasts don’t cross routers.
What are IPv6 Router Advertisements (RAs)?
IPv6 routers automatically send out ICMPv6 Router Advertisement (RA) messages.
These messages help hosts (like PCs) figure out how to get their IPv6 address and other settings.
A host can dynamically be assigned an address using stateless or stateful services.
What is meant by Stateless and Stateful in IPv6 configuration?
Stateless: No device is tracking the assignment of IPv6 addresses.
- SLAAC
- SLAAC with DHCP Server (Stateless DHCPv6).
Stateful: A DHCPv6 server is managing the assignment of IPv6 addresses.
- DHCPv6 Server (Stateful DHCPv6).
What is SLAAC?
SLAAC is a stateless service which means there is no server that maintains network address information to know which IPv6 addresses are being used, and which ones are available.
The IPv6 address and default gateway come from the RA (Router Advertisement) message.
The device generates its own IPv6 address using:
- The prefix from the RA.
- Its own interface ID (often based on MAC address or randomly generated).
What is SLAAC with DHCP Server (Stateless DHCPv6)?
SLAAC: The device self-generates its IPv6 address using the network prefix in the router advertisement (RA).
DHCPv6: Used only to get extra info like DNS, domain name, etc. The DHCPv6 server only provides configuration parameters for clients and does not maintain a list of IPv6 address bindings (i.e. it is stateless).
TLDR: “I’ll make my own IP address but ask DHCP for other stuff.”
What is DHCPv6 Server (Stateful DHCPv6)?
The DHCPv6 server gives the full IPv6 address and other settings (like DNS). The DHCPv6 server is stateful and maintains a list of IPv6 address bindings.
Works just like IPv4 DHCP.
The router tells the device: “Ask the DHCPv6 server for everything.”
What are the three RA message flags? What do they do?
A (Autoconfiguration) = Use SLAAC to create your own IPv6 address.
O (Other info) = Use DHCPv6 for extras (like DNS).
M (Managed) = Use DHCPv6 to get everything, including your address.
Using different combinations of the A, O and M flags, the RA messages inform the host about the dynamic options available.
SLAAC:
- A = 1, O = 0, M = 0
Stateless DHCP:
- A = 1, O = 1, M = 0
Stateful DHCP:
- A = 0, M = 1
What happens if both the ‘M’ and ‘A’ flags are set to “1” in an RA?
M = 1: The host must use DHCPv6 to get a full IPv6 address and other network settings.
A = 1: The host may also generate its own IPv6 address using SLAAC with the prefix from the RA.
Depending on the operating system and settings, the host might:
- Use both addresses simultaneously (DHCPv6 + SLAAC).
- Prefer one address over the other.
This provides flexibility and redundancy in address assignment.
How do you enable RA messages on a router?
A router must have IPv6 routing enabled before it can send RA messages: “ipv6 unicast-routing”.
A router sends RA messages every 200 seconds or when it receives an RS message from a host.
Describe how a PC obtains an IPv6 address through SLAAC
- PC sends a Router Solicitation (RS) message to request configuration information from the local router.
- Router replies with a Router Advertisement (RA) message that includes the network prefix and flags.
- PC generates its own IPv6 address by combining the advertised prefix with its interface identifier (usually based on the MAC address or a random value).
- PC performs Duplicate Address Detection (DAD) by sending a Neighbor Solicitation to check if the generated IPv6 address is already in use on the network.
- If no response is received, the PC considers the address unique and assigns it to its interface.
What is Duplicate Address Detection (DAD)?
A SLAAC host may use the Duplicate Address Detection (DAD) process to ensure that the generated IPv6 GUA is unique.
However, DAD is really not required because a 64-bit interface ID provides 18 quintillion possibilities, and the chance of a duplicate address is remote
How do you enable Stateless DHCPv6 on a router?
Stateless DHCPv6 is enabled using the “ipv6 nd other-config-flag” interface configuration command on R1, to set the O flag to 1.
How do you enable Stateful DHCPv6 on a router?
Stateful DHCPv6 is enabled using the “ipv6 nd managed-config-flag” interface configuration command, to set the M flag to 1.
Additionally, manually change the A flag from 1 to 0 using the “ipv6 nd prefix default no-autoconfig” command
What are the advantages of Stateful DHCPv6 over Stateful DHCPv6?
Keeps track of all IPv6 addresses it assigns to devices.
Allows network admins to control and filter which devices can get addresses.
Provides better traceability for security and auditing.
Why is Stateful/Stateless DHCP less common with IPv6?
Because IPv6 devices can use SLAAC (Stateless Address Autoconfiguration) to automatically generate their own IP addresses without needing a DHCP server.
