Administration réseaux

This assessment concerns network administration and tests knowledge and understanding of DHCP, DNS, IP addressing, and related network protocols. It includes multiple-choice questions and a practical exercise on DNS query resolution, requiring both conceptual understanding and applied reasoning.

QCM Question 1

What is being asked: Explain how a DHCP server dynamically assigns an IP address to a host.

Step-by-step reasoning:

• Option A suggests that the address is chosen after negotiation with the host, which is incorrect because DHCP assigns addresses from a pool without host negotiation on the specific address.
• Option B states addresses are assigned for a limited time, after which a new search and assignment occur. This aligns with the DHCP lease concept.
• Option C says addresses are leased and the host typically keeps the same address by renewing the lease periodically, which accurately describes DHCP behavior.
• Option D claims addresses are permanent, which contradicts the dynamic nature of DHCP.

Final answer: The correct description is C. The addresses are leased to the host, which usually keeps the same address by periodically renewing the lease with the DHCP server.

QCM Question 2

What is being asked: Identify the two tasks performed by a DHCP server.

Step-by-step reasoning:

• Option A says the server discovers hosts using DHCPDISCOVER messages, but DHCPDISCOVER is sent by clients, not servers.
• Option B states the server configures IP parameters for hosts, which is a core DHCP function.
• Option C mentions monitoring IP performance using DHCP, which is not a DHCP server task.
• Option D says the server assigns and renews IP addresses from a pool, which is correct.

Final answer: The two correct tasks are B. Configuring IP parameters from the DHCP server to a host and D. Assigning and renewing IP addresses from the default pool.

QCM Question 3

What is being asked: Identify the correct statement about DHCP operation.

Step-by-step reasoning:

• Option A claims a DHCP client uses ping to detect address conflicts. This is true: clients often ping an address before using it to detect conflicts.
• Option B says the DHCP server uses ARP to detect clients, which is incorrect; servers do not detect clients via ARP.
• Option C says clients use ARP to detect servers, which is false; clients discover servers via broadcast DHCP messages.
• Option D says that if a conflict is detected, the address is removed from the pool and another is offered, which is a correct DHCP conflict resolution step.

Between A and D, both are plausible, but the question asks for a correct statement about DHCP operation. Since clients do use ping (ICMP echo) to detect conflicts before accepting an address, and the server removes conflicted addresses, both are partially correct. However, the best-known DHCP client behavior is to ping before accepting an address.

Final answer: A. A DHCP client uses ping to detect address conflicts.

QCM Question 4

What is being asked: Identify which application layer protocol is responsible for the failure to access www.uvt.rnu.tn by name, while access via IP works.

Step-by-step reasoning:

• The problem is that typing the IP works, but the domain name does not. This indicates a failure in name resolution.
• DHCP (A) is about IP address assignment, not name resolution.
• DNS (B) is the protocol that resolves domain names to IP addresses.
• HTTP (C) and HTTPS (D) are protocols for web communication but do not handle name resolution.

Final answer: B. DNS is responsible for the failure.

QCM Question 5

What is being asked: Identify the top-level domain (TLD) in the URL "http://www.cisco.com/netacad.html".

Step-by-step reasoning:

• The full domain is www.cisco.com.
• The top-level domain is the last part after the final dot, which is ".com".
• Options:
• A. Cisco.com - this is the second-level domain plus TLD.
• B. .com - this is the top-level domain.
• C. www.Cisco.com - full hostname, not just TLD.
• D. www - a subdomain.
• E. netacad - a path or subdirectory, not a domain.

Final answer: The top-level domain is B. .com.

QCM Question 6.1

What is being asked: After the DHCP server fails, what happens immediately to hosts on the same network as PC1 that had DHCP leases?

Step-by-step reasoning:

• Hosts with valid DHCP leases keep their IP addresses until the lease expires.
• Since the server just failed, leases are still valid, so hosts continue to communicate normally for some time.
• They can communicate within and outside their network as long as their IP configuration is valid.

Final answer: B. The hosts will continue to communicate normally for a certain time.

QCM Question 6.2

What is being asked: For hosts on PC2's network with static IPs communicating normally, but PC2 failing, what solutions could be adopted? Choose two.

Step-by-step reasoning:

• Since PC2 uses DHCP and cannot communicate, the issue is likely DHCP relay or routing.
• Option A suggests correcting R3's routing table to forward requests to R1 and R2, which could help DHCP requests reach the server.
• Option B suggests placing a DHCP relay server on PC2's network, which helps forward DHCP requests across routers.
• Option C suggests transferring DHCPDISCOVER requests to R2, which may be a DHCP server, but this depends on network design.
• Option D suggests R3 must be RFC 1542 compliant and configured accordingly, which is necessary for DHCP relay agents.

Among these, the two best solutions are:

• A. Correct the routing table of R3 to forward requests to R1 and R2.
• B. Place a DHCP relay server on PC2's network.

Exercice

What is being asked: For a host in the domain machin.com making a DNS request to resolve the IP of the web server at toutenbas.endessous.labas.tn, list the DNS queries and their types sent to the necessary DNS servers, noting that only the host can send recursive queries.

Step-by-step reasoning:

The host wants the IP address of toutenbas.endessous.labas.tn. The domain hierarchy is:

• toutenbas (subdomain)
• endessous (subdomain)
• labas (subdomain)
• tn (top-level domain)

The host is in machin.com, so it does not have direct authority over the tn domain. The host sends a recursive query to its configured DNS server (likely the machin.com DNS server). The recursive DNS server will perform iterative queries on behalf of the host.

The queries and types are as follows:

1. The host sends a recursive DNS query (type A) for toutenbas.endessous.labas.tn to its local DNS server.
2. The local DNS server queries the root DNS server (type NS) to find the authoritative server for the .tn domain.
3. The local DNS server queries the .tn DNS server (type NS) to find the authoritative server for labas.tn.
4. The local DNS server queries the labas.tn DNS server (type NS) to find the authoritative server for endessous.labas.tn.
5. The local DNS server queries the endessous.labas.tn DNS server (type NS) to find the authoritative server for toutenbas.endessous.labas.tn.
6. Finally, the local DNS server queries the toutenbas.endessous.labas.tn DNS server (type A) to get the IP address.
7. The local DNS server returns the IP address to the host.

Summary of queries:

• Host to local DNS server: recursive A query for toutenbas.endessous.labas.tn
• Local DNS server to root server: iterative NS query for .tn
• Local DNS server to .tn server: iterative NS query for labas.tn
• Local DNS server to labas.tn server: iterative NS query for endessous.labas.tn
• Local DNS server to endessous.labas.tn server: iterative NS query for toutenbas.endessous.labas.tn
• Local DNS server to toutenbas.endessous.labas.tn server: iterative A query for IP address

Final answer: The host sends one recursive A query to its DNS server. The DNS server performs iterative NS queries down the domain hierarchy from root to toutenbas.endessous.labas.tn, finally retrieving the A record and returning it to the host.

Method

This paper rewards clear understanding of DHCP and DNS protocols, especially the dynamic IP leasing process, lease renewal, and conflict detection in DHCP, as well as the hierarchical nature of DNS queries and the distinction between recursive and iterative queries.

It punishes confusion between client and server roles in DHCP message exchanges, misunderstanding of domain name hierarchy, and failure to explain reasoning steps clearly. Correct identification of protocol functions and the ability to trace network communication flows are essential. Partial knowledge without explanation or skipped steps reduces credit.