Summary
Highlights
The application layer of the TCP/IP model corresponds to the application, presentation, and session layers of the OSI model. The application layer (OSI) handles user interface, the presentation layer formats/encrypts/compresses data, and the session layer manages communication dialogues between sender and receiver.
Network models include client-server, where a client requests services from a server (e.g., webpages, emails), and peer-to-peer (P2P), where devices act as both clients and servers, directly connecting to each other. Examples of P2P applications include BitTorrent and Direct Connect.
Accessing a webpage involves six steps: the user types a URL (e.g., cisco.com), the PC queries DNS servers for the IP address, the DNS server provides the IP, the client sends an HTTP command to that IP, the web server replies with the index.html file, and the browser displays the page. HTTP secures data in clear text, while HTTPS uses encryption for secure transport.
Email protocols include Simple Mail Transfer Protocol (SMTP) for sending emails, and Post Office Protocol version 3 (POP3) or Internet Message Access Protocol (IMAP) for retrieving emails. POP3 deletes emails from the server after retrieval, while IMAP keeps a copy on the server.
DNS includes top-tier domains like .com, .net, .edu, and country-specific domains. Second-level domains like 'cisco.com' are organized under these. DNS records map names to IP addresses: 'A' records for IPv4, 'AAAA' records for IPv6, 'NS' for name servers, and 'MX' for mail exchange records. All DNS records are stored in DNS zones.
The 'nslookup' command is a utility for querying DNS servers to resolve hostnames. It's crucial for troubleshooting name resolution issues and verifying the status of name servers.
A DHCP server leases IP addresses and configuration to hosts for a specific interval. If not renewed, the IP returns to the pool. DHCP uses a four-step handshake (DHCP Discover, Offer, Request, ACK) between client (port 68) and server (port 67). Clients use RARP to request IP addresses for known MACs. DHCP allows for reserved addresses for specific devices (e.g., printers) and the exclusion of certain IP addresses from the leasing pool, especially for statically configured devices like default gateways.