TCP/IP Questions and Answers

Answer: B

A client that sends out a DHCP Discover message in order to receive an IP address sends out a broadcast at both layer 2 and layer 3. The layer 2 broadcast is all Fs in hex, or FF:FF:FF:FF:FF:FF. The layer 3 broadcast is 255.255.255.255, which means all networks and all hosts. DHCP is connectionless, which means it uses User Datagram Protocol (UDP) at the Transport layer, also called the Host-to-Host layer.

Answer: D

Correct answer: DNS

DNS uses UDP port 53 for ordinary queries, because a query and its reply are small and a lost one is simply retried, and TCP port 53 when the response exceeds what a single datagram will carry and for zone transfers between servers. FTP, SMTP and Telnet all use TCP alone.

Answer: C

Correct answer: Class C

A Class C network uses 24 bits for the network and 8 for the host, giving \( 2^8 = 256 \) addresses of which two are reserved - the all-zeros network address and the all-ones broadcast - leaving 254 usable hosts. Class B gives 65,534 and Class A gives 16,777,214, while Class D is multicast and has no host portion at all.

Answer: B

Correct answer: Host-to-Host

The DoD model’s four layers map onto the OSI seven as follows: Application covers OSI layers 5 to 7, Host-to-Host corresponds to Transport, Internet to Network, and Network Access to Data Link and Physical. Host-to-Host is where TCP and UDP live, which is the practical way to remember it.

Answer: A

Correct answer: Application

Telnet and FTP are application layer protocols, and the Application layer is the highest in the OSI model, so that is the highest layer in use. The question is asking which layer the protocol itself occupies rather than which layers the data passes through - it passes through all seven on the way down.

Answer: B

Correct answer: 2, 3 and 5

SMTP, HTTP and FTP all use TCP, because each needs delivery guaranteed and in order - a mail message, a web page and a file transfer are all useless with pieces missing. DHCP and TFTP use UDP: DHCP because a client has no address yet and must broadcast, and TFTP because it implements its own simple acknowledgement scheme and was designed to be small enough to fit in boot ROM.

Answer: C

Correct answer: 3 and 4

Statements 3 and 4 are correct: ICMP reports network problems - destination unreachable, time exceeded, redirects - and its messages travel encapsulated inside IP datagrams. Statement 1 is wrong, since acknowledging TCP segments is TCP’s own job. Statement 2 is wrong because ICMP reports failures rather than preventing them; IP remains an unreliable, best-effort service whatever ICMP says about it.

Answer: C

Correct answer: Internet

The Internet layer of the DoD model corresponds to the Network layer of OSI, and it is where IP, ICMP, ARP and RARP operate - logical addressing and routing between networks. Host-to-Host is Transport, one layer above it, and Network Access covers the two OSI layers below.

Answer: B

Correct answer: ARP

ARP resolves a known IP address to the hardware address on the local segment, by broadcasting a request that every host sees and only the owner answers. RARP does the reverse, obtaining an IP address from a known hardware address, and ICMP reports errors rather than resolving anything. The result is held in the ARP cache, which show ip arp displays.

Answer: C

Correct answer: 3, 4 and 5

Telnet, FTP and TFTP are all application layer protocols. IP is at the Internet layer and TCP at the Host-to-Host layer, so neither belongs at the Application layer - they are what the application protocols run over. Note that TFTP counts here despite using UDP: the transport it chooses does not change the layer it occupies.