OSI and TCP/IP Models with Device and Protocol Mapping
Key Takeaways
- The OSI model is a troubleshooting map: physical, data link, network, transport, session, presentation, and application.
- The TCP/IP model groups similar functions into link, internet, transport, and application layers.
- Devices and protocols are often identified by the layer where their primary decision happens.
- Encapsulation adds headers as data moves down the stack and removes them as data moves up the stack.
- Layer mapping helps narrow symptoms, choose tools, and avoid replacing the wrong component.
OSI as a Troubleshooting Map
Network+ uses the OSI model to organize symptoms. It is not just a memorization list. If the link light is dark, start low. If IP works but names fail, look higher. If TCP connects but the application rejects credentials, the network path may be fine.
| OSI layer | Primary job | Common examples | Useful clues |
|---|---|---|---|
| 7 Application | User-facing network services | HTTP, HTTPS, DNS, DHCP, SMTP, SNMP, SSH | Service error, name failure, application timeout |
| 6 Presentation | Format, encryption, encoding | TLS, certificates, compression | Certificate warning, unsupported cipher |
| 5 Session | Session setup and control | RPC sessions, NetBIOS session concepts | Session drops, authentication conversation issues |
| 4 Transport | End-to-end ports and reliability | TCP, UDP, port numbers | Connection refused, retransmits, blocked port |
| 3 Network | Logical addressing and routing | IPv4, IPv6, ICMP, routers, Layer 3 switches | Wrong gateway, no route, TTL exceeded |
| 2 Data Link | Frames, MAC addresses, VLANs | Ethernet, switches, 802.1Q, ARP, Wi-Fi MAC | VLAN mismatch, MAC table, duplex issue |
| 1 Physical | Signaling and media | Cables, fiber, radio, connectors, transceivers | No link, bad cable, interference, attenuation |
TCP/IP Model Mapping
| TCP/IP layer | Rough OSI mapping | Examples |
|---|---|---|
| Application | OSI 5-7 | DNS, DHCP, HTTP, HTTPS, SMTP, SSH, SNMP |
| Transport | OSI 4 | TCP, UDP |
| Internet | OSI 3 | IPv4, IPv6, ICMP, routing |
| Link | OSI 1-2 | Ethernet, Wi-Fi, ARP, switching, cabling |
Device and Protocol Mapping
| Item | Primary layer | Why |
|---|---|---|
| Hub | 1 | Repeats electrical or optical signals without frame decisions |
| Switch | 2 | Forwards frames using MAC addresses |
| Router | 3 | Forwards packets between IP networks |
| Layer 3 switch | 2/3 | Switches frames and routes between VLANs |
| Firewall | 3/4/7 depending on type | Filters by IP, port, state, or application |
| Load balancer | 4 or 7 | Distributes sessions by transport or application data |
| DNS | 7 | Resolves names through an application-layer service |
| TCP | 4 | Provides connection-oriented transport with ports |
| UDP | 4 | Provides connectionless transport with ports |
| ARP | 2/3 boundary | Resolves IPv4 addresses to MAC addresses on a local network |
Scenario: Narrow by What Works
A desktop has a link light, receives a DHCP lease, can ping its gateway, but cannot access https://intranet.example by name.
| Evidence | What it suggests |
|---|---|
| Link light is on | Layer 1 is likely functional |
| DHCP lease exists | Layer 2 path to DHCP or relay worked, and basic IP settings exist |
| Gateway ping works | Local Layer 3 path is working |
| Name-based website fails | DNS, application service, certificate, or HTTP/HTTPS path needs checking |
Layer thinking prevents a cable replacement from becoming the first answer to a DNS clue.
Encapsulation Names
| Layer area | Data unit |
|---|---|
| Application data | Data |
| Transport | Segment for TCP, datagram for UDP |
| Network | Packet |
| Data link | Frame |
| Physical | Bits or symbols |
When data leaves a host, each lower layer adds information needed for delivery. When data arrives, the receiving stack removes and interprets those headers.
A switch forwards traffic based primarily on MAC addresses. Which OSI layer is most associated with that decision?
Match each item to its primary OSI layer.
Match each item on the left with the correct item on the right
Order the encapsulation units as data moves down the stack from an application toward the wire.
Arrange the items in the correct order