Networking basics and the internet

Dez Okafor-Laine, the network and systems administrator, keeps a whiteboard in the technology office with a hand-drawn map of the library's network: fourteen branches, the central library, the bookmobile's cellular link, and a cloud marked "internet" at the top. When Noor started in October, the map looked like spaghetti. Now Lorna has asked Dez to walk her through it, because a large share of help desk tickets are really network tickets in disguise: "the catalog is slow," "the kiosk says offline," "I can't print," "the website won't load." Tech+ expects you to identify basic networking concepts and compare the common types of internet service. Dez's version is shorter: "Know what each box on the map does, and know how a request gets from a public computer to a website and back."

What a network is

A network is two or more devices connected so they can share data and resources. The devices, called nodes or hosts, include computers, phones, printers, servers, kiosks and cameras. A local area network, or LAN, covers one site, such as a single branch library, a home or an office floor. A wide area network, or WAN, connects LANs across distances, such as the links between Fairhaven's branches and the central library. The internet is the largest WAN of all: a global network of networks run by thousands of organizations that agree to carry one another's traffic using the same protocols.

Data on a network travels in small pieces called packets. A large file or web page is split into many packets, each carrying a piece of the data plus addressing information that says where it came from and where it is going. Packets may take different routes and arrive out of order; the receiving computer puts them back together. A protocol is an agreed set of rules for communicating. The internet runs on a family of protocols called TCP/IP. IP, the Internet Protocol, handles addressing and routing packets between networks. TCP, the Transmission Control Protocol, makes sure packets arrive complete and in order, and resends any that are lost. UDP is a lighter alternative used where speed matters more than perfection, such as video calls and online games.

The devices on the map

A switch connects devices within a LAN. Each computer, printer or access point plugs into a switch port with an Ethernet cable, and the switch forwards each packet only to the port where its destination is connected, using the hardware MAC address of each device. A router connects different networks and decides where to send packets that are leaving the local network; every branch has a router that links its LAN to the library's WAN and the internet. A wireless access point lets Wi-Fi devices join the LAN without a cable, and is covered in lesson 7. A modem converts between the signals used by the internet provider's line, such as cable or DSL, and the Ethernet the router understands; fiber services use a similar box often called an optical network terminal. A firewall filters traffic between networks according to rules, allowing what is needed and blocking the rest. In a home, a single box often combines a modem, router, switch, access point and firewall, which is why people call the whole thing "the router."

Addresses: IP, MAC and ports

Every device on a network needs an address. A MAC address is a hardware identifier built into each network adapter, written as six pairs of hexadecimal digits, such as 3C-52-82-1A-7F-04; switches use it to deliver packets inside a LAN. An IP address is a logical address assigned to a device on a network, and routers use it to deliver packets between networks. IPv4 addresses are four numbers from 0 to 255 separated by dots, such as 192.168.20.45, which gives about four billion possible addresses. That is not enough for every device in the world, so IPv6 was created, with much longer addresses written in hexadecimal groups separated by colons, such as 2001:db8:4a2:20::45. Both are in use today, side by side.

To stretch the supply of IPv4 addresses, networks use private address ranges inside their LANs: 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255 and 192.168.0.0 to 192.168.255.255. These addresses are not used on the public internet. When a device with a private address reaches the internet, the router translates it to the router's own public address, a process called network address translation, or NAT. That is why every computer at a branch can browse the web while the outside world sees only one or a few public addresses. A device that shows an address beginning 169.254 has given itself an automatic private address because it could not get one from the network, which usually points to a cable, Wi-Fi or DHCP problem.

A port number identifies which service on a device a packet is meant for, so one server can run a website, email and file sharing at once. Well-known ports include 80 for HTTP web traffic, 443 for HTTPS secure web traffic, 53 for DNS, 25 and 587 for sending email, 22 for SSH secure remote administration, and 3389 for Windows Remote Desktop. Firewalls often allow or block traffic by port.

import ipaddress
for text in ['192.168.20.45', '10.4.0.12', '172.20.1.9', '8.8.8.8', '169.254.8.20']:
    ip = ipaddress.ip_address(text)
    if ip.is_link_local:
        kind = 'self-assigned: no DHCP answer'
    elif ip.is_private:
        kind = 'private: used inside a LAN'
    else:
        kind = 'public: used on the internet'
    print(text.ljust(15), kind)

192.168.20.45   private: used inside a LAN
10.4.0.12       private: used inside a LAN
172.20.1.9      private: used inside a LAN
8.8.8.8         public: used on the internet
169.254.8.20    self-assigned: no DHCP answer

def is_private_v4(text):
    parts = [int(p) for p in text.split('.')]
    return False

DHCP and DNS

Two services make networks easy to use. DHCP, the Dynamic Host Configuration Protocol, hands out IP addresses automatically. When a laptop joins the branch Wi-Fi, it asks for an address, and a DHCP server, usually built into the router, replies with an address, the subnet information, the router's address (called the default gateway) and the DNS servers to use. Without DHCP, every device would need its address typed in by hand. Printers, servers and other devices that others must find reliably are often given a reserved or static address that does not change.

DNS, the Domain Name System, translates names that people remember, such as fairhavenlibrary.example, into the IP addresses computers use. When a patron types a web address, the computer asks a DNS server for the address, then connects to it. DNS works like a phone book that is spread across thousands of servers around the world. When DNS fails, websites appear to be down even though the internet connection works, because the computer cannot find out where to go. A common test is to reach a site by its IP address; if that works but the name does not, DNS is the problem.

Types of internet service

Each branch needs a connection to an internet service provider, or ISP, and Tech+ expects you to compare the main types. Fiber-optic service carries data as light through glass fibers and offers the highest speeds, often a gigabit or more in both directions, with low latency; most city branches use fiber. Cable internet uses the same coaxial cable network as cable television; it is fast for downloads but usually slower for uploads, and speeds can drop at busy times because neighbors share capacity. DSL, the Digital Subscriber Line, uses telephone lines; it is slower, and speed falls the farther the building is from the telephone exchange. Cellular service uses 4G LTE or 5G mobile networks; it needs no cable at all, which is why the bookmobile and the library's lending hotspots use it, but data plans may be capped and signal varies. Fixed wireless uses a dish or antenna on the building pointed at a nearby tower, common in rural areas.

Satellite internet reaches places no cable does. Traditional satellites sit in geostationary orbit about 36,000 kilometers above the equator, so every signal travels a very long way and latency is high, often over half a second, which makes video calls awkward. Newer low-earth-orbit services use large numbers of satellites only a few hundred kilometers up, which cuts latency to levels close to cable. Speed is not the only measure: Tech+ expects you to weigh bandwidth (how much data per second), latency (how long a round trip takes), reliability, availability at the location, data caps and cost.

Bandwidth, latency and what users feel

Users experience the network through two qualities that are easy to confuse. Bandwidth is capacity, measured in bits per second: a wider pipe moves more data at once, so large downloads finish sooner. Latency is delay, measured in milliseconds: the time for a packet to reach its destination and for a reply to come back. A connection can have high bandwidth and high latency at the same time, like a wide pipe that is very long. Video streaming mostly needs bandwidth; video calls, online games and remote desktop sessions mostly need low latency. A speed test website reports both, usually as download speed, upload speed and ping time.

Noor phones the bookmobile driver, who reports one bar of signal. The driver moves the van fifty meters up the road to the top of a slight rise, the router picks up a stronger signal, and the catalog loads. Dez adds a note to the bookmobile's route sheet showing where signal is strong at each stop, and Lorna asks Noor to write a two-line knowledge base article: "Connected to Wi-Fi does not mean connected to the internet."