Connect the services.
Follow the protocols.

Separate application protocols from transport

Application protocols define message formats and procedures used by applications. They rely on transport-layer services such as TCP or UDP and network-layer addressing to communicate between hosts. In the OSI model, the Presentation layer handles data representation, formatting, compression, and encryption/decryption; the Session layer establishes, maintains, and recovers dialogs; and the Application layer provides network services to end-user applications. TCP/IP groups these three OSI layers into its Application layer.

Resolve names and configure hosts

DNS translates domain names to IP addresses so users and applications do not need to memorize numeric addresses. A recursive resolver follows referrals from root servers to a top-level-domain server and then to the domain's authoritative name server. A maps a name to IPv4, AAAA to IPv6, MX identifies mail exchangers (lower preference values are preferred), and NS identifies authoritative name servers. A delegation may need glue A/AAAA records when the name server is inside the delegated zone. DHCPv4 automatically assigns IP configuration using Discover, Offer, Request, Acknowledgment (DORA).

Recognize common application services

HTTP and HTTPS support web communication. SMTP sends email; POP3 and IMAP retrieve it. FTP transfers files, TFTP provides simple UDP-based transfers, and SMB supports shared files and printers. The application protocol determines the exchange; TCP or UDP carries the transport data.

Compare client/server and peer-to-peer

In a client/server model, a client requests a service and a server responds. A peer-to-peer application allows each peer to act as a client for one request and a server for another. A pure P2P design does not require a central directory. A hybrid P2P design may use a central index to locate a resource while peers transfer the actual content directly.

Follow a web and email exchange

For an HTTP/1.1 request by hostname, the browser resolves the name, establishes the transport connection (and TLS for HTTPS), sends an HTTP request, receives a response with a status and content, and renders the returned resource. GET retrieves a representation, POST submits data for processing, and PUT creates or replaces the target resource representation. SMTP is used for sending and relay: TCP 25 is commonly used for server-to-server relay, while authenticated client submission commonly uses TCP 587 with STARTTLS or TCP 465 with implicit TLS. POP3 commonly uses TCP 110 and IMAP TCP 143; secure variants commonly use 995 and 993. POP3 clients can be configured to leave messages on the server, so deletion is not an unavoidable protocol property.

Inspect DNS answers and run nslookup

A recursive resolver follows the DNS hierarchy: root servers refer it to a TLD, TLD servers refer it to authoritative servers for a zone, and the authoritative server returns the requested record. The resolver may cache answers for the resource-record TTL. A zero-TTL answer may be returned for the current query but should not be retained for later reuse as a fresh cached record. Use nslookup to query the configured resolver and inspect A, AAAA, MX, and NS answers. MX records include a preference value; lower values are preferred. NS records name the authoritative server, while glue records provide an address when needed to reach a delegated server.

nslookup www.example.com
nslookup -type=MX example.com
nslookup -type=NS example.com

Record the DNS server used and inspect the answer, authority, and additional sections when present. An IP connection can succeed while hostname access fails, so test name resolution separately from basic IP reachability.

Understand DHCPv4 and DHCPv6

DHCPv4 uses DORA: Discover, Offer, Request, Acknowledgment. A client without an IPv4 address initially broadcasts DHCPDISCOVER: Ethernet source is the client's own MAC, Ethernet destination is FF:FF:FF:FF:FF:FF, IPv4 source is 0.0.0.0, IPv4 destination is 255.255.255.255, and UDP ports are client 68 to server 67. The server replies from UDP 67 to client 68. DHCPv6 uses different message names and UDP ports 546 (client) and 547 (server). A common stateful exchange is SOLICIT, ADVERTISE, REQUEST, REPLY; a host using SLAAC can send INFORMATION-REQUEST and receive REPLY for other configuration such as DNS.

Choose a file-sharing service

FTP uses a control connection (commonly TCP 21) and a separate data connection. In classic active mode, the server's data connection commonly uses TCP 20; passive mode negotiates a server-side data port, so TCP 20 is not universal. TFTP uses UDP 69 for its initial request, then uses negotiated transfer identifiers/ports for the data exchange. SMB supports authenticated sessions and shared files/printers. These protocols have different feature sets and transport behavior; they are not interchangeable.