2 Core 1: Networking

Network types and layers

A network connects devices so they can exchange data and share services. Networks vary by their reach and purpose:

• PAN: a personal-area network, such as a phone connected to earbuds over Bluetooth.

• LAN: a local-area network, typically within a home, office, or building.

• WLAN: a LAN that uses wireless connections.

• MAN: a metropolitan-area network connecting sites across a city or region.

• WAN: a wide-area network connecting distant networks; the internet is the largest example.

• SAN: a dedicated network that gives servers access to shared storage.

Network communication uses layered protocols. The OSI model is a reference framework for organizing these functions: physical media and signals are at Layer 1; local delivery and MAC addresses are at Layer 2; IP addressing and routing are at Layer 3; and TCP or UDP transport is at Layer 4. Troubleshooting often involves identifying the failing layer. For example, a damaged cable is a physical-layer problem, while an incorrect IP address is a network-layer problem.

Addressing and configuration

An IPv4 address identifies a network interface. Its subnet mask, or prefix length, identifies which part of the address belongs to the local network. Devices on the same subnet can usually communicate directly; traffic for another subnet is sent to the default gateway, usually a router.

DNS translates host names such as example.com into IP addresses. DHCP automatically leases network settings, commonly including an IP address, subnet mask, gateway, and DNS server. A static configuration is entered manually, while a dynamic configuration is usually supplied by DHCP.

If a device configured for automatic addressing cannot contact DHCP, it may self-assign an IPv4 address in 169.254.0.0/16, a range associated with APIPA. This typically permits only limited local communication. An address such as 169.254.x.x commonly indicates DHCP configuration failure or unavailability. IPv6 addresses are longer and written in hexadecimal groups separated by colons.

Private IPv4 ranges used inside networks are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. A router commonly uses NAT to translate private internal addresses for internet access. NAT is not, by itself, a complete security control.

An incorrect static address, subnet mask, gateway, or DNS server can prevent connectivity even when the cable or Wi-Fi connection is working.

Protocols and common ports

A port identifies an application service at the transport layer. The numbers below are common defaults, not proof of which service is actually using a port. IANA maintains the registry of service names and port numbers.

• FTP: TCP ports 20 and 21; file transfer, with 21 traditionally used for control and 20 for data.

• SSH: TCP port 22; encrypted remote command-line access.

• Telnet: TCP port 23; unencrypted remote command-line access and insecure.

• SMTP: TCP port 25; email transfer between mail servers.

• DNS: UDP and TCP port 53; name resolution, with TCP used for some larger responses and transfers.

• DHCP: UDP ports 67 for the server and 68 for the client; automatic IPv4 configuration.

• TFTP: UDP port 69; simple file transfer, often for device boot or configuration.

• HTTP: TCP port 80; unencrypted web traffic.

• POP3: TCP port 110; email retrieval.

• NTP: UDP port 123; time synchronization.

• IMAP: TCP port 143; email access and synchronization.

• SNMP: UDP ports 161 for queries and 162 for traps; network-device monitoring and alerts.

• LDAP: TCP or UDP port 389; directory-service access.

• HTTPS: TCP port 443; web traffic protected by TLS. HTTP/3 can use UDP on port 443.

• SMB: TCP port 445; file and printer sharing, especially on Windows networks.

• RDP: TCP or UDP port 3389; Remote Desktop access.

TCP establishes a connection and provides ordered, reliable delivery. UDP has less delivery overhead and is often used for short queries, streaming, or time-sensitive traffic. A firewall can filter traffic by address, protocol, and port; allow only required services and use secure alternatives where possible.

Cabling and connectors

Twisted-pair copper Ethernet cable contains paired conductors to reduce interference. UTP is unshielded; STP adds shielding for environments with electrical interference. Common Ethernet connectors are 8P8C modular plugs, often called RJ-45.

Coaxial cable has a central conductor and shielding and is used in some broadband and cable-TV connections. Fiber-optic cable carries light and is resistant to electromagnetic interference. Single-mode fiber is suited to longer distances; multimode fiber is commonly used over shorter links.

T568A and T568B specify the wire order in twisted-pair terminations. A straight-through cable uses the same standard at both ends. A crossover cable uses a different standard at each end. Many modern ports automatically adapt, but cabling should follow device and cabling requirements.

A patch panel terminates and organizes permanent cable runs. Short patch cables connect the panel to a switch or connect nearby equipment. Choose cable type and rating for the environment. A cable tester can check continuity and wiring faults; a toner and probe can help trace a cable. Avoid sharply bending fiber and observe laser-safety precautions when working with optical equipment.

Wireless networking

Wi-Fi is a family of IEEE 802.11 wireless LAN standards. Devices communicate over radio frequencies, commonly in the 2.4 GHz, 5 GHz, and, on supported equipment, 6 GHz bands. The 2.4 GHz band often reaches farther but can be more congested. Higher-frequency bands can offer more capacity but may have less range through obstacles. Actual speed and coverage depend on the standard, channel width, interference, distance, and client capabilities.

An SSID is the wireless network name. A channel is a portion of the radio spectrum used for communication. Nearby networks, walls, appliances, and poor access-point placement can reduce performance. A Wi-Fi analyzer can help identify channel congestion and weak coverage.

Use WPA3 where supported, or WPA2 with AES when needed for compatibility. Avoid obsolete protections such as WEP. A strong, unique passphrase and updated access-point firmware improve security; hiding the SSID is not a substitute for encryption.

Network devices and segmentation

Network devices have distinct roles, though home equipment may combine several functions:

• A modem or ONT connects a customer network to an ISP’s service—commonly cable or DSL for a modem, and fiber for an ONT.

• A router forwards traffic between IP networks and often provides NAT, DHCP, firewall, and basic Wi-Fi functions in home devices.

• A switch connects devices on a LAN and forwards Ethernet frames toward the appropriate port. Managed switches can support features such as VLANs.

• An access point (AP) lets wireless clients join a wired LAN. A Wi-Fi router commonly combines an AP, router, switch, and other functions.

• A firewall permits or blocks traffic according to security rules. It may be a separate device, software, or a feature of a router.

• A PoE switch or injector supplies power over Ethernet to compatible equipment such as APs and IP cameras.

• A repeater/extender relays a signal to improve coverage, often with some performance trade-offs.

A VLAN logically separates devices into distinct Layer 2 networks on supported switches. Communication between VLANs requires routing. A VPN creates an encrypted tunnel across an untrusted network to a private network or service.

Configuration and troubleshooting

Use a consistent troubleshooting sequence: check power and link status, inspect cables or Wi-Fi association, verify the device’s address settings, test local connectivity, and then test name resolution and remote services. Change one variable at a time and retest.

Useful Windows commands include:

ipconfig /all displays adapter configuration. /release and /renew release and request DHCP settings. ping tests IP reachability using ICMP, but a failed ping is not conclusive because firewalls may block ICMP. tracert shows responding hops along a route; some routers do not respond, so missing hops do not necessarily indicate failure. nslookup tests DNS resolution.

If a computer can reach the local gateway by IP but cannot open websites by name, the local link and basic IP path are working. Test DNS with nslookup; if name resolution fails, check the configured DNS server and whether it is reachable. If a computer instead has a 169.254.x.x address, check its link and whether it can reach a DHCP server.

For poor Wi-Fi performance in a crowded office, a Wi-Fi analyzer can reveal channel congestion. Interference, distance, or obstacles may also contribute. More generally, verify the physical or wireless link first, then IP settings and gateway reachability, followed by DNS and the target service.