9 Network Practice and Troubleshooting

Learn how network layers, protocols, addresses, devices, and troubleshooting tests work together to diagnose connectivity problems.

Use layers to classify network functions

The provides a useful map for classifying network functions and narrowing down which kind of fault to investigate. Internet protocols do not always fit neatly into one layer, so use the model as a practical guide rather than an exact description of every protocol.

Layer 7, Application: Network services used by applications, including HTTP, , , and SMTP.
Layer 6, Presentation: Data representation, encryption, and compression, such as TLS encryption and character encoding.
Layer 5, Session: Starting, managing, and ending communication sessions, including session-control functions in applications.
Layer 4, Transport: Process-to-process delivery, ports, and reliable or datagram-based transport, including and .
Layer 3, Network: Logical addressing and forwarding between networks, including IPv4, IPv6, , and routers.
Layer 2, Data Link: Local-link frames and hardware addressing, including Ethernet, Wi-Fi link functions, switches, and MAC addresses.
Layer 1, Physical: Signals, connectors, and transmission media, including copper, fiber, radio, repeaters, and hubs.

In the /IP model, application functions commonly span OSI layers 5–7, transport corresponds roughly to layer 4, Internet to layer 3, and link to layers 1–2. This mapping is practical, not an exact one-to-one translation.

How carries data

A browser request begins as application data. may place it in a segment, IP carries that segment in a packet, and Ethernet or Wi-Fi carries the packet in a local-link frame. The receiving system processes the layers in reverse. A forwards IP packets between networks; it does not normally forward the original local Ethernet frame unchanged.

Match services with protocols and ports

A port identifies a transport-layer endpoint used by an application or service, but a port number alone does not prove which application is using it. These are common defaults, not guarantees: a service can be configured to use another port. IANA maintains the registry of assigned service names and ports.

: Port 53 over and ; resolves names and other information.
server and client: Ports 67 and 68 over ; assigns network configuration to clients.
SSH: Port 22 over ; encrypted remote login and command access.
HTTP: Port 80 over ; web traffic without transport encryption.
HTTPS: Port 443 over ; HTTP protected with TLS. HTTP/3 uses QUIC over .
SMTP: Port 25 over ; mail transfer between mail servers.
NTP: Port 123 over ; clock synchronization.

provides a reliable, ordered byte stream. provides datagrams without ’s delivery guarantees. , used by ping, does not use or ports.

Interpret addresses and subnet ranges

An identifies a network-layer interface and helps deliver traffic across networks. A supports delivery on a local link. A is the a host uses to reach destinations outside its local subnet. A or prefix indicates which IP bits identify the subnet.

Private IPv4 address ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These addresses are for private networks and are not globally routed as public Internet addresses.

Worked example: `192.168.10.77/26`

The /26 prefix means 26 network bits and 6 host bits. The corresponding mask is 255.255.255.192. Each subnet contains 64 addresses because $2^6 = 64$ .

The ranges in the last octet increase by 64: .0–.63, .64–.127, .128–.191, and .192–.255. Since .77 falls within .64–.127, the network address is 192.168.10.64, the broadcast address is 192.168.10.127, and usable host addresses run from 192.168.10.65 through 192.168.10.126.

For ordinary IPv4 subnets like this one, network and broadcast addresses are not assigned to hosts. The /26 notation is classless prefix notation: it specifies the number of leading network bits.

Recognize devices, media, and architectures

Devices have different roles in moving traffic. A connects devices on a local network and commonly forwards Ethernet frames using MAC addresses. A Layer 2 operates primarily at OSI layer 2; a multilayer can also route at layer 3. A connects IP networks and forwards packets toward destinations using IP routes. Home routers often combine routing, switching, Wi-Fi, and firewall functions.

An connects wireless clients to a network, commonly bridging Wi-Fi clients to a wired LAN. A hub or repeater regenerates or repeats signals at the physical layer; unlike a , a basic hub does not make forwarding decisions based on MAC addresses. A modem or ONT connects a customer network to a provider’s access technology, and its exact role depends on the service.

Common network media include twisted-pair copper, fiber-optic cable, and radio used by Wi-Fi. Copper and fiber carry signals through cable, while Wi-Fi carries them over radio. Wired Ethernet and Wi-Fi can be part of the same LAN, but wireless clients share the radio capacity of their access point.

Network architecture describes how systems are arranged or how they provide services. In a client-server design, clients request services from servers. In peer-to-peer communication, devices can provide services directly to one another. A star topology connects devices through a central or AP; a mesh provides multiple interconnections or paths. These distinctions can help locate a central-device failure or identify whether alternate paths exist.

Follow a layered troubleshooting sequence

Troubleshoot from the local connection outward, changing one variable at a time. This sequence helps separate local-link, configuration, routing, name-resolution, and application problems.

Physical and link: Check power, link lights, cable seating, Wi-Fi association, and whether the network adapter is enabled.
Local configuration: Inspect the , or prefix, , and server. Confirm that the address belongs to the intended subnet and is not duplicated.
Local network: Test communication with the host’s gateway. If this fails, investigate the host link, VLAN or Wi-Fi network, address configuration, and gateway interface.
Remote IP connectivity: Try reaching a known remote . If the gateway responds but remote IP traffic fails, investigate routing, firewall rules, or the provider path. A failed ping is not conclusive because may be filtered.
Name resolution and application: If IP connectivity works but names fail, test . If names resolve but one application fails, check its service, port, firewall policy, and server status.
Trace the path: Use traceroute or, on Windows, tracert to observe where responses stop. Intermediate devices may not respond, so a missing hop alone does not prove that the device or path is broken.

Useful commands include ipconfig on Windows, ip addr on many Linux systems, ping, tracert or traceroute, and nslookup or dig. Interpret their results as evidence rather than absolute proof: firewalls, routing policy, and host configuration can affect what a test reports. Layered diagnosis is effective because Internet communication uses cooperating protocols across link, IP, transport, and application functions.

Apply the concepts to common symptoms

Use the layer, address, protocol, and device clues in each situation to choose a sensible first check.

Wi-Fi signal is present, but the laptop has an address beginning 169.254 and cannot reach its gateway: This is a in 169.254.0.0/16. The computer may not have received configuration. Check Wi-Fi association, the AP connection, service, and the client’s address, mask, and gateway. Link-local addresses can support local-link communication but do not by themselves provide ordinary routed connectivity.
A computer can reach 192.168.10.65 but not a server on another subnet: The local IP path works. Check the host’s and , then routes and the remote network. A successful local ping does not prove that the or destination network is reachable.
A website is reachable by but not by its domain name: This points toward name resolution. Test the configured server and the name lookup. commonly uses port 53 over or ; reaching a server by IP is separate from resolving its name.
A cable is connected, but the port has no link light: Start at the physical layer. Inspect the cable, connector, endpoint network interface, and port. For fiber, check that compatible optics are installed and that transmit and receive strands are correctly paired.
A technician asks for the common SSH port and whether ping uses it: SSH commonly uses port 22. ping uses , not a or port.