Networking Basics

Names such as LAN and WAN describe scope and interconnection, but exact size boundaries and ownership are not universal technical cutoffs.

• LAN (Local Area Network): Connects devices over a limited local scope, such as a home, office, or campus network. Its speed, ownership, and security depend on the equipment, service, and configuration.

• WAN (Wide Area Network): Connects networks or sites over a broader geographic scope, often using services and infrastructure from multiple providers. It is not inherently slower or less secure than a LAN; those properties depend on the actual links and controls.

• WLAN (Wireless LAN): A LAN whose local links use wireless technologies such as Wi-Fi. A WLAN may also connect to wired networks through an access point or bridge.

• MAN (Metropolitan Area Network): A term sometimes used for a network spanning a metropolitan area. The label describes scope broadly; it does not prescribe one medium, owner, or performance level.

• VPN (Virtual Private Network): A logical private connection carried over another network. VPN technologies can use encryption and authentication, but a VPN label alone does not guarantee security; protocol choice, configuration, endpoints, and access policy matter.

Topology describes how nodes and links are arranged physically or logically. The resulting failure behavior depends on the actual graph, devices, link redundancy, and any routing or protection mechanism.

• Star: Endpoints connect through a central hub, switch, or access device. A failed endpoint link isolates that endpoint; failure of the central device affects endpoints that depend on it. Redundant devices or links can change the outcome.

• Bus: Nodes share a common communication path in a bus arrangement. The medium and access protocol determine how transmissions are shared; a break or termination fault can affect many nodes. This is a topology description, not a claim about current industrial-network prevalence.

• Ring: Links form a closed logical cycle. Traffic direction and recovery depend on the ring protocol and implementation; redundant rings can reroute around some failures.

• Mesh: A full mesh connects each node to every other node; a partial mesh connects selected node pairs. An alternate path exists only where links create one, and traffic uses it only if routing or protection selects it. Cost and resilience depend on the specific design.

Devices implement different functions at the link and Internet layers. Product boundaries can combine several of these roles.

• Ethernet switch (bridge): Connects devices within a bridged LAN or VLAN. It learns source MAC addresses and uses a forwarding table to send frames toward the destination link. Broadcast and unknown-destination frames may be flooded within the configured VLAN; a switch does not always send a frame only to one port.

• IP router: Forwards IP packets between networks using the destination IP prefix and forwarding information derived from routes. Route selection is handled by routing and forwarding mechanisms; a router does not independently calculate an end-to-end Internet path for every packet.

• Modem: Modulates and demodulates signals for a communications medium, such as a cable or telephone service. Signal formats depend on the access technology; the term does not mean that every network link carries analog signals.

• Network interface: Hardware or software that lets a host use a network link. Ethernet interfaces use link-layer addresses such as MAC addresses, which may be factory-assigned, locally administered, virtual, or changed by configuration; they are not guaranteed to be permanent identifiers for a person or device.

Layer models group communication functions to help describe protocol responsibilities. The OSI reference model defines seven layers. Internet protocol stacks are often taught using four broad layers. The correspondence is useful for orientation, but it is not a strict one-to-one mapping for every protocol.

4.1 The 7-Layer OSI (Open Systems Interconnection) Model

• 7. Application: Protocol services used by applications, such as HTTP, DNS, and email protocols.
• 6. Presentation: Describes data representation and syntax. Encryption may be provided by protocols that do not fit neatly in one OSI layer.
• 5. Session: Describes dialogue and session coordination; Internet applications may handle these functions in different ways.
• 4. Transport: Provides communication between application endpoints. TCP supplies a reliable ordered byte stream; UDP supplies datagrams without delivery or duplicate-protection guarantees. Ports identify transport endpoints.
• 3. Network: Provides logical addressing and packet forwarding across networks. IP routers forward using destination prefixes and routes.
• 2. Data Link: Provides communication over a local link using that link's frame and address rules, such as Ethernet MAC addressing.
• 1. Physical: Carries symbols or bits through a medium such as copper, optical fiber, or radio.

4.2 The 4-Layer TCP/IP Model

One common four-layer Internet view groups functions as follows:

• Application: Application-level protocols and data representation, broadly corresponding to OSI layers 5–7.
• Transport: Host-to-host application communication, including TCP and UDP, broadly corresponding to OSI layer 4.
• Internet: IP addressing and forwarding between networks, broadly corresponding to OSI layer 3.
• Link: Delivery over an attached link and its medium, broadly corresponding to OSI layers 1–2.

5.1 IPv4 vs. IPv6

• IPv4: A 32-bit address written as four decimal octets (for example, 192.0.2.10). The format has 2³² possible bit patterns; some blocks have special uses, and available public allocation is limited.

• IPv6: A 128-bit address written as up to eight hexadecimal groups separated by colons. Leading zeroes may be omitted and consecutive zero groups may be compressed (for example, the documentation address 2001:db8::10). IPv6 provides a much larger address space than IPv4, not an infinite one.

5.2 Subnetting and CIDR Notation

An IPv4 subnet mask marks the prefix bits used for a network block. In CIDR notation, the slash suffix gives the number of leading prefix bits; for example, /24 means 24 prefix bits and 8 remaining host bits in IPv4. Address planning and routing use these boundaries. A worked /26 example appears below.

5.3 NAT (Network Address Translation)

Network Address Translation (NAT) changes address information as packets cross a network boundary. A common IPv4 configuration also translates transport ports (often called NAPT or PAT), allowing multiple internal address-and-port pairs to share an external IPv4 address. NAT behavior varies; it does not by itself provide a firewall policy or solve IPv4 address scarcity.

A protocol defines message formats and behavior that communicating systems agree to follow. Protocols can be standardized or private, and each protocol provides only the services specified for it.

• HTTP and HTTPS: HTTP defines request and response semantics used by web services. HTTPS commonly carries HTTP over TLS to protect a connection and authenticate a server when certificates are verified. HTTP/3 carries the same HTTP semantics over QUIC, which uses TLS 1.3; the transport differs by HTTP version.

• DNS (Domain Name System): A distributed naming system queried for resource records associated with domain names. A and AAAA records can provide IPv4 and IPv6 addresses. A URL contains more than a domain name, and DNS does not translate the entire URL.

• DHCP (Dynamic Host Configuration Protocol): A client can request an address lease and configuration options such as a subnet mask, router, or DNS server from a DHCP service. Leases expire or renew under protocol rules; networks can instead use static configuration.

• TCP (Transmission Control Protocol): Provides a reliable, in-order byte stream between endpoints. It uses sequence numbers, acknowledgments, and retransmission to recover from some loss; a connection can still fail, and successful transport does not prove that an application completed its work. HTTP/1.1 and HTTP/2 commonly use TCP.

• UDP (User Datagram Protocol): Provides datagram delivery with minimal transport mechanisms; delivery, ordering, and duplicate protection are not guaranteed. Applications can add recovery or ordering when needed. UDP is not inherently faster in every use; HTTP/3, for example, uses QUIC over UDP and adds transport features above UDP.

• ARP (Address Resolution Protocol): For IPv4 over Ethernet, resolves a next-hop protocol address to a local Ethernet address. A host resolves its gateway's address when sending off-link; a router resolves the next hop on its outgoing link. IPv6 uses Neighbor Discovery instead of ARP.

• FTP (File Transfer Protocol): A client-server protocol for transferring files. The base FTP protocol does not provide transport encryption; use an appropriately secured alternative where confidentiality is required.

• SMTP, POP3, and IMAP: SMTP is used to submit or relay email. POP3 and IMAP are mailbox-access protocols with different retrieval and synchronization behavior.

• ITU-T Recommendation X.200, OSI Basic Reference Model
• IEEE 802.1Q, Virtual LANs and VLAN Bridges
• IETF RFC 791, Internet Protocol, RFC 1122, Requirements for Internet Hosts, and RFC 4632, Classless Inter-domain Routing
• IETF RFC 8200, IPv6 Specification, RFC 3849, IPv6 Documentation Address Prefix, and RFC 9637, Expanded IPv6 Documentation Prefixes
• IETF RFC 826, Ethernet Address Resolution Protocol
• IETF RFC 3022, Traditional NAT, RFC 3021, /31 IPv4 Prefixes on Point-to-Point Links, and RFC 5737, IPv4 Documentation Address Blocks
• IETF RFC 768, User Datagram Protocol, RFC 8085, UDP Usage Guidelines, and RFC 9293, Transmission Control Protocol
• IETF RFC 1034, Domain Names—Concepts and Facilities and RFC 1035, Domain Names—Implementation and Specification
• IETF RFC 2131, Dynamic Host Configuration Protocol
• IETF RFC 8446, TLS 1.3, RFC 9110, HTTP Semantics, and RFC 9114, HTTP/3
• IETF RFC 4861, IPv6 Neighbor Discovery
• IETF RFC 959, File Transfer Protocol, RFC 2577, FTP Security Considerations, RFC 5321, Simple Mail Transfer Protocol, RFC 1939, POP3, and RFC 9051, IMAP4rev2