Computer systems and networks · GCSE Computer Science
TCP/IP layers
GCSE Computer Science TCP/IP: application, transport, internet and network-access layers, what each one is for, and how a packet is passed down and back up the stack.
Four layers: application (HTTP, DNS), transport (TCP or UDP ports), internet (IP addressing and routing), network access (frames, MAC, Wi-Fi or Ethernet). Data is chopped into packets and handed down.
The important bits
What you need to know
- 1
The TCP/IP model has four layers: application, transport, internet (network), and network access (link). Each layer adds its own header and has a job the others should not duplicate.
- 2
Application layer: the meaning for the user or program. HTTP/HTTPS for web pages, DNS for name lookup, FTP for files, SMTP/IMAP for email. This is not “the app store”.
- 3
Transport layer: end-to-end communication between processes. TCP is reliable, ordered, with retransmission; UDP is lightweight with no delivery guarantee. Port numbers identify the service (e.g. 443 for HTTPS).
- 4
Internet layer: IP addresses and routing between networks. Routers look at the destination IP and choose the next hop. ICMP (ping) lives here in many descriptions.
- 5
Network access / link layer: getting a frame onto the local medium. MAC addresses, Ethernet, Wi-Fi, network interface. Switches work with frames on a LAN; they are not routing across the internet.
- 6
Encapsulation: as data goes down the stack, each layer wraps the payload. Going up, headers are stripped. A packet is the internet-layer unit; papers also say “packets” for the whole journey — match the question’s word.
- 7
DNS sits at the application layer: names to IP addresses so the internet layer has somewhere to route. If DNS fails, the site can be up and still unreachable by name.
- 8
Layers stop one giant rulebook. You can change Wi-Fi for Ethernet (network access) without rewriting HTTP (application). That independence is the point of the model.
Quotations worth analysing
Short evidence. Real method.
“Application, transport, internet, network access.”
Learn this order, top to bottom. Mixing in all seven OSI names unless the paper asks is wasted time and often wrong.
“TCP provides reliable transmission; IP is responsible for addressing and routing packets.”
TCP is transport; IP is internet layer. “TCP/IP” as one blob loses those two marks. Reliability is not the same as routing.
“Data is broken into packets for transmission.”
Packets can take different routes and are reassembled. That is why a missing packet can be resent by TCP without resending the whole file.
Go deeper
A URL walk is a four-layer story
You type a URL. Application: the browser builds an HTTP(S) request and asks DNS (also application) for the IP. Transport: TCP opens a connection to port 443, splits the request into segments, and will later resend anything lost. Internet: each packet gets a destination IP; routers hop it across networks. Network access: on your LAN the packet is wrapped in a frame with the gateway’s MAC address and sent over Wi-Fi or Ethernet. At the server the layers unwrap in reverse: frame off, IP checked, TCP reassembled, HTTP handled. If you skip DNS, the internet layer has no address. If you confuse MAC with IP, you have mixed network access with internet. That sequence is a 6-mark gift if you keep the order and name one protocol per layer.
Go deeper
Packets, reliability, and what a layer must not steal
Packet switching chops a message so links can be shared and a lost chunk can be retried. TCP on the transport layer numbers segments and retransmits; IP on the internet layer just tries to get each packet closer to the destination IP and does not promise order. That split answers “why both TCP and IP”. UDP exists when speed matters more than completeness (some games, video). Do not put HTTP on the internet layer or IP on the application layer. Network access is allowed to talk about collision, bandwidth, and MAC; it is not where you explain HTTPS certificates. HTTPS is application data wrapped in TLS, still sitting above TCP. For performance questions, bandwidth and errors often live on the link; latency can be routing (internet) or congestion (many layers). Name the layer that matches the factor you were asked for.
See the idea in action
Send “GET /index.html” to a web server at 93.184.216.34 from a school PC. Application: HTTP request; DNS has already resolved the name to 93.184.216.34. HTTPS would encrypt here / just above TCP. Transport: TCP, destination port 443, source port chosen by the PC. The request is segmented. Sequence numbers will detect loss. Internet: IP header, destination 93.184.216.34, source = the PC’s IP (or the router’s after NAT). Each packet is routed independently. Network access: the PC sends a frame to the default gateway’s MAC via the switch/WAP. Fibre or copper or radio carries the bits. A router strips the frame, reads the IP, forwards a new frame onto the next link. Only the internet layer’s IP address survives that hop unchanged (until NAT). Server stack reverses: frame → IP → TCP reassembly → HTTP response packets back down. If you wrote “the switch uses IP to find the website”, you used the wrong layer. Switches: MAC, network access. Routers: IP, internet.
Exam technique
Turn knowledge into marks
Memorise four layers and one protocol for each. In a “what happens when you enter a URL” question, walk down the stack: DNS and HTTP, then TCP ports, then IP routing, then MAC/frame on the local network.
Common mistakes
Do not give these marks away
- 01
Putting HTTP on the internet layer, or saying a switch routes using IP addresses.
- 02
Treating TCP and IP as the same thing, or claiming IP retransmits lost packets (that is TCP).
- 03
Listing seven OSI layers when the paper asked for TCP/IP, or forgetting that data is split into packets.
Which TCP/IP layer is responsible for IP addressing and routing packets between networks?
AApplication layer
BTransport layer
CInternet layer
DNetwork access layer
Show the answer
Internet layer. IP addresses and routers live at the internet layer. Transport is TCP/UDP and ports. Application is HTTP/DNS. Network access is frames and MAC addresses on the local link.
Quick questions
If this is the bit you searched
What are the four layers of TCP/IP GCSE?
Application, transport, internet, and network access (sometimes called the link layer). Data moves down the stack to send and up the stack to receive.
What is the difference between TCP and IP?
TCP (transport) provides reliable, ordered delivery with ports and retransmission. IP (internet layer) addresses packets and routes them between networks. Together they are the TCP/IP suite.
Which layer uses MAC addresses?
Network access (link). MAC identifies an interface on the local network. IP identifies a host across networks. A switch forwards using MAC; a router forwards using IP.
Why is data split into packets?
So large messages can share the network, take different routes, and be resent in pieces if something is lost. TCP can request missing packets without restarting the whole download.