3.10Networks (TCP/IP, IP Addresses, Wireless LAN, and Communication Speed)

Sharing files in a company office, or connecting to the internet from home to browse the web—both are possible because of the networking mechanisms that connect multiple computers. This section covers networking fundamentals end to end: the familiar distinction between LAN and WAN, TCP/IP as the foundation of internet communication, IP addresses that identify devices, and the frequently tested topic of calculating communication speed.

3.10.1LAN/WAN, TCP/IP, and communication devices

• LAN (Local Area Network) connects devices within a limited area, such as an office or home. WAN (Wide Area Network) connects geographically separated sites using a carrier's network lines (the internet is a type of WAN).
• TCP/IP is the standard set of communication protocols used on the internet. Devices from different manufacturers can communicate as long as they follow the same common rules. A router is a device that relays traffic between different networks (e.g., connecting a home LAN to the internet). A hub is a device that connects multiple devices within the same LAN.

3.10.2IP addresses, DNS, DHCP, wireless LAN, and email

• An IP address is an address-like number that identifies a device on the network (e.g., 192.168.1.1). DNS (Domain Name System) is the name-resolution mechanism that maps human-friendly domain names (www.example.com, etc.) to IP addresses. DHCP automatically assigns an IP address when a device connects to a network, saving the trouble of manual configuration.
• Wireless LAN (Wi-Fi) connects to a LAN over radio waves without cables. Browsing the web uses HTTP/HTTPS; sending email uses SMTP; receiving email from a mail server uses POP (downloads to the device and generally deletes it from the server) or IMAP (manages mail on the server, letting multiple devices see the same state).
• Communication speed calculation finds transfer time from a line's theoretical speed (bps, bits per second), the file size (in bytes), and transmission efficiency. The keys are remembering that 1 byte = 8 bits, and that effective speed = theoretical speed x transmission efficiency.

Consider the network setup of a newly opened cafe to see how the pieces connect. Inside the shop, several POS registers and tablets form a LAN, connected via a hub (or switch), while a router sits at the internet gateway, relaying to the WAN that is the internet. Each device is assigned an IP address automatically via DHCP, sparing staff the trouble of manual configuration. When a customer connects to the shop's wireless LAN (Wi-Fi) and browses a website, the browser queries a DNS server for the IP address corresponding to a domain name like www.example.com, then communicates with that IP address following TCP/IP rules. A reservation-confirmation email is sent to the mail server via SMTP, and the shop's device retrieves it from the server via POP or IMAP (if multiple staff members want to check the same mail status from their own phones rather than sharing one device, IMAP, which keeps state on the server, is the better fit). Now consider downloading 200 megabytes of data from the shop's cloud inventory system. If the line's theoretical speed is 100 Mbps and transmission efficiency is 80%, the effective speed is 100 Mbps x 0.8 = 80 Mbps. Since 200 megabytes (MB), multiplied by 8, becomes 1600 megabits (Mb), the transfer time works out to roughly 1600 Mb / 80 Mbps = 20 seconds. Forgetting the byte-to-bit conversion throws the answer off by a factor of 8, making it the easiest mistake in this type of calculation problem.

3.10.3Section summary

• LAN = limited area; WAN = wide area. Router = relays between different networks; hub = connects devices within the same LAN
• DNS = translates domain names and IP addresses; DHCP = auto-assigns IP addresses. Wireless LAN connects without cables
• SMTP = sending; POP/IMAP = receiving. For speed calculations, remember the 1 byte = 8 bit conversion and multiplying by transmission efficiency