Which one of the following protocols is not an application layer of the TCP/IP model?

Correct — D, IP. The TCP/IP model has four layers — application, transport, internet and network access. FTP moves files, HTTP carries web pages and DNS turns a domain name into an address, and all three are services a user's program calls on directly, so all three sit at the application layer. IP does something different. It belongs to the internet layer, where its job is to give every host a logical address and to route packets from one network to the next, best-effort and without any guarantee of delivery. Every application-layer protocol in the list ultimately travels inside IP packets.

• (a)FTP — The File Transfer Protocol is an application-layer service — a user or a program invokes it directly to move files between hosts, and it relies on TCP beneath it.
• (b)HTTP — The Hypertext Transfer Protocol is what a browser speaks to a web server, which places it squarely at the application layer.
• (c)DNS — The Domain Name System resolves names such as an ordinary web address into numeric addresses. It is a lookup service used by applications, and it too runs at the application layer.

The TCP/IP model stacks network functions in four layers. The application layer holds the protocols a user's software speaks — HTTP, FTP, DNS, SMTP, Telnet. The transport layer, with TCP and UDP, delivers data between processes and adds reliability where it is wanted. The internet layer, with IP at its centre, addresses hosts and routes packets across interconnected networks. The network access layer handles the physical link.

The wording rewards attention. Three of the four options are named by what they let a user do — transfer a file, fetch a page, look up a name — while IP is named by what it does for the network itself. That functional test is more reliable than memorising lists: if a protocol exists so that an application can accomplish something, it is at the application layer; if it exists so that packets can find their way, it is lower down. It also helps to remember that the layers are nested, not parallel. An HTTP request is carried by TCP, which is carried by IP, which is carried over the physical link, so IP is not an alternative to HTTP but the road beneath it. The older OSI model splits the same job into seven layers and maps its top three onto the single application layer used here.

• The TCP/IP model has four layers — application, transport, internet and network access — against the seven of the OSI model.
• HTTP, FTP, DNS, SMTP and Telnet are application-layer protocols.
• TCP and UDP are transport-layer protocols; TCP adds reliable, ordered delivery, UDP does not.
• IP works at the internet layer, providing logical addressing and routing; it is connectionless and best-effort.
• An IPv4 address is 32 bits long and an IPv6 address 128 bits, which is why the newer version was needed as address space ran short.

• Assuming that anything ending in 'P' for protocol belongs to the same layer.
• Treating IP as an alternative to HTTP rather than as the layer that carries it.
• Mixing the four TCP/IP layers with the seven OSI layers when the question names one model.

Asked as an odd-one-out on protocol layers, with three application-layer services set against one internet-layer protocol.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following works at the transport layer of the TCP/IP model?

  • (a)HTTP
  • (b)TCP
  • (c)ARP
  • (d)SMTP

  Answer(b) TCP — the transport-layer protocol that provides reliable, ordered delivery; HTTP and SMTP are application layer and ARP sits below.

• practice — not a real PYQ

  The protocol that translates a domain name into a numeric address is

  • (a)FTP
  • (b)DNS
  • (c)IP
  • (d)UDP

  Answer(b) DNS — the Domain Name System, an application-layer lookup service.