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Networking concepts explained, with examples from a Mac.
IP and addressing 24
Subnet Cheat Sheet (CIDR, Masks, Host Counts)
Every IPv4 prefix length on one page: subnet mask, wildcard, total and usable addresses, and how it compares with a /24.
What Is a MAC Address?
A MAC address identifies a network interface on the local network. Learn its format, what the first half reveals, and why phones and Macs now randomize it.
What Is a Subnet? Subnetting Explained
A subnet is a range of IP addresses that forms its own network. Learn how subnet masks define them, why networks are split, and how to read one on your Mac.
What Is an IP Address?
An IP address is the number that identifies a device on a network so data can reach it. Learn the kinds of addresses, how your devices share one public IP, and how to find yours.
CIDR Notation Explained
CIDR notation writes a network as an address and a prefix length, like 192.168.1.0/24. The number after the slash says how many bits are the network part.
DNS vs DHCP
DHCP sets your device up on the network; DNS finds the addresses of everything you connect to. DHCP even tells your device which DNS server to use.
IPv4 vs IPv6: What's the Difference?
IPv4 has 4.3 billion addresses and relies on NAT; IPv6 has a practically unlimited supply and gives every device a global address. Most devices now use both.
LAN vs WAN
A LAN is the local network in your home or office; a WAN connects networks over long distances. Your router sits between the two.
Public vs Private IP Address
Private IP addresses identify devices inside your network; the public IP identifies your whole network on the internet. Your router translates between them.
Static vs Dynamic IP Address
A dynamic IP is handed out automatically and can change; a static IP stays the same. Most devices should be dynamic; servers and printers often need static.
Unicast vs Multicast vs Broadcast
Unicast sends to one device, multicast to a group that asked for it, and broadcast to every device on the local network. Most traffic is unicast.
What Is a Default Gateway?
Your default gateway is the router that forwards traffic from your network to everywhere else. Here's how your Mac uses it and how to find its address.
What Is a Link-Local Address?
Link-local addresses work only on the local network segment. In IPv4 (169.254.x.x) they signal a DHCP problem; in IPv6 (fe80::) every interface has one.
What Is a Loopback Address?
A loopback address points back at your own computer. Traffic sent to 127.0.0.1 or ::1 never leaves the machine.
What Is a Subnet Mask?
A subnet mask tells a device which part of an IP address is the network and which is the device. Here's how to read one, with 255.255.255.0 as the example.
What Is an ASN (Autonomous System Number)?
An ASN identifies a network that runs its own routing on the internet, like an ISP, a cloud provider or a big company. Every public IP belongs to one.
What Is Anycast?
With anycast, the same IP address lives in many places at once, and the internet sends you to the nearest. 1.1.1.1 and 8.8.8.8 work this way.
What Is ARP?
ARP translates a local IP address into the hardware (MAC) address needed to actually deliver a frame on Ethernet or Wi-Fi.
What Is BGP?
BGP is how the internet's networks share routes with each other. It decides the path your traffic takes between ISPs, clouds and companies.
What Is CGNAT?
With carrier-grade NAT, your ISP shares one public IP address among many customers. It breaks port forwarding and hosting from home.
What Is DHCP?
DHCP is why your Mac gets online the moment it joins Wi-Fi: the router hands it an IP address and everything else it needs.
What Is Dynamic DNS?
Dynamic DNS keeps a host name pointing at your home's public IP even when your ISP changes it, so you can reach home devices by name.
What Is IPv6?
IPv6 is the newer version of the Internet Protocol, with 128-bit addresses that let every device have a global address. Here's how it works and how to see it on your Mac.
What Is NAT?
NAT lets all the devices in your home share one public IP address by rewriting addresses as traffic passes through the router.
DNS 9
What Is DNS? How Domain Names Work
DNS translates human-friendly names like apple.com into the IP addresses computers use. Every website visit starts with a DNS lookup.
DNS Record Types Explained (A, AAAA, CNAME, MX, TXT, PTR)
A domain's DNS zone holds several kinds of records: addresses, aliases, mail servers, text and more. Here's what each type does.
What Is a DNS Server?
A DNS server turns domain names into IP addresses. There are two kinds: resolvers that answer your devices, and authoritative servers that hold the records.
What Is a Domain Name?
A domain name is the readable address you type instead of an IP address. It's rented from a registrar and pointed at servers with DNS.
What Is a URL? Parts of a URL Explained
A URL is the web address of a page or file. Learn its parts, what each one does, and what happens when you press Return.
What Is Bonjour on Mac?
Bonjour is how your Mac discovers printers, AirPlay speakers and shared drives without any setup, using .local names and multicast DNS.
What Is DNS over HTTPS (DoH)?
DNS over HTTPS sends your DNS lookups inside encrypted HTTPS connections, so Wi-Fi operators and ISPs can't read or tamper with them.
What Is DNSSEC?
DNSSEC adds digital signatures to DNS so resolvers can verify that answers really come from the domain's owner and weren't tampered with.
What Is TTL in Networking?
TTL means two different things: a hop counter that stops packets looping forever, and the time a DNS answer may be cached.
Ports and protocols 23
Common Network Ports: The Complete List
The ports you'll actually run into, from 22 to 51820, with the protocol, what uses them and where they show up on a Mac.
HTTP Status and Error Codes: The Complete List
Every HTTP response starts with a three-digit status code. Here's what each class and common code means, with links to fixes for the errors you'll actually see.
What Is TLS? (SSL vs TLS, Certificates)
TLS encrypts and authenticates connections: it's the S in HTTPS and protects email, APIs and most other internet traffic. SSL is its obsolete predecessor.
HTTP vs HTTPS
HTTPS is HTTP sent through an encrypted, authenticated TLS connection. It stops the network from reading or changing what you send and receive.
TCP vs UDP: What's the Difference?
TCP sets up a connection and guarantees delivery in order; UDP sends independent packets with no guarantees. Each suits different apps.
The OSI Model Explained (All 7 Layers)
The OSI model splits networking into seven layers, from cables to apps. It's a vocabulary more than a blueprint, but it's how engineers talk about problems.
The TCP Three-Way Handshake
Every TCP connection starts with three packets, SYN, SYN-ACK and ACK, that agree on sequence numbers before any data flows.
The TCP/IP Model Explained
The TCP/IP model describes the internet in four layers, from cables and Wi-Fi up to apps. Each layer has one job and relies on the one below.
What Is a CDN?
A CDN keeps copies of a site's files on servers around the world, so each visitor downloads from somewhere nearby. Most of your Mac's connections go to one.
What Is a Network Packet?
Networks move data in small chunks called packets, each with headers that say where it comes from and where it's going. Here's what's inside one.
What Is a Port in Networking?
A port is a number that tells a device which app should receive incoming data. Learn how ports work with IP addresses, which numbers are standard, and what's open on your Mac.
What Is a Socket?
A socket is the endpoint an app uses for network communication. Learn how sockets relate to IP addresses and ports, and how to see them on your Mac.
What Is a WebSocket?
A WebSocket is a connection that stays open, so the server and browser can send each other messages at any moment. It powers chat, live dashboards and multiplayer games.
What Is FTP?
FTP is one of the internet's oldest protocols for moving files between computers. It's unencrypted, so today SFTP or FTPS usually replace it.
What Is ICMP?
ICMP is the internet's messaging protocol for errors and diagnostics. Ping and traceroute are built on it.
What Is MTU?
MTU is the largest packet a link can carry. When a path has a smaller MTU than expected, small pages load but large transfers hang.
What Is NTP?
NTP keeps computer clocks accurate to within milliseconds by syncing with time servers. Here's why that matters and how your Mac does it.
What Is Port Forwarding?
Port forwarding lets devices on the internet reach a specific device on your home network, for game servers, remote access or self-hosting.
What Is QUIC?
QUIC is a modern transport protocol that runs over UDP, builds in TLS 1.3 encryption, and powers HTTP/3. It makes connections faster and more resilient.
What Is RDP?
RDP lets you see and control a Windows computer over the network. Macs connect with Microsoft's free Windows App.
What Is SMTP?
SMTP is how email is sent: from your mail app to your provider, and between mail servers. Reading mail uses other protocols, IMAP or POP.
What Is SNMP?
SNMP lets monitoring systems read counters and status from network devices like routers, switches and printers, and receive alerts from them.
What Is SSH?
SSH gives you an encrypted command line on another computer, and secure file transfer and tunnels. Here's how the connection works and how keys protect it.
Performance 8
Bandwidth vs Throughput
Bandwidth is what your connection could carry; throughput is what it actually carries. The gap comes from Wi-Fi, latency, loss and the far end.
Latency vs Bandwidth (and Ping)
Bandwidth is the width of the pipe; latency is its length. Most of what feels slow day to day is latency, not bandwidth.
What Is Bandwidth?
Bandwidth is the maximum data rate of a connection. Learn how it's measured, how it differs from actual speed and latency, and how much you need.
What Is Bufferbloat?
Bufferbloat is when your latency shoots up as soon as someone uploads or downloads. Calls stutter and games lag even though the speed test looks fine.
What Is Jitter?
Jitter is how much your latency varies from packet to packet. Steady latency is fine for calls; jumpy latency makes them break up.
What Is Latency?
Latency is the delay between sending data and getting a response. It's why a fast connection can still feel sluggish in calls and games.
What Is Packet Loss (and What Causes It)?
Packet loss is when some of the packets your device sends or receives never arrive. A little causes stutter and slowdowns; a lot makes connections unusable.
What Is Ping?
Ping checks whether a device answers on the network and measures the round-trip time. It's also what gamers mean by their connection's ping.
Security and privacy 7
What Is a VPN and How Does It Work?
A VPN encrypts your traffic and sends it through a server elsewhere, hiding it from your local network and changing the IP address websites see.
What Is a Firewall?
A firewall decides which network connections are allowed. Your router has one, your Mac has one, and they protect against different things.
What Is a Proxy Server?
A proxy server forwards your requests to websites and returns the answers, so the site sees the proxy instead of you. It's narrower and simpler than a VPN.
What Is a Reverse Proxy?
A reverse proxy is the front door of a website: it receives every request and passes it to the right server behind it.
What Is a VLAN?
A VLAN splits one physical network into several separate networks, so devices on the same switch can be kept apart. Here's how and why.
What Is a VPN Tunnel?
A VPN tunnel wraps your packets inside encrypted packets addressed to the VPN server, so the network in between sees only the wrapper.
WireGuard vs OpenVPN
WireGuard is a newer, much smaller VPN protocol that's usually faster; OpenVPN is older, highly configurable and can hide in HTTPS traffic. Both are secure.
Wi-Fi and routers 7
2.4 GHz vs 5 GHz Wi-Fi (and Wi-Fi 6E)
2.4 GHz reaches further; 5 GHz is faster with less interference; 6 GHz (Wi-Fi 6E and 7) is faster still at short range. Here's how to choose.
Router vs Modem: What's the Difference?
The modem connects your home to your internet provider; the router builds your home network and shares that connection. Many ISP boxes do both.
Switch vs Router (and Hub, Access Point)
A switch connects devices within one network; a router connects different networks, like your home and the internet. Access points add Wi-Fi; hubs are obsolete.
What Is a Router?
A router connects networks together. At home it links your devices to the internet, and quietly does half a dozen other jobs too.
What Is an SSID?
An SSID is the name of a Wi-Fi network: what you pick from the list when you connect. Here's what it does and what it doesn't.
What Is Ethernet?
Ethernet is the standard for wired networks. It's usually faster, steadier and lower-latency than Wi-Fi. Here's how it works and how to use it on a Mac.
What Is Mesh Wi-Fi?
Mesh Wi-Fi uses several units working together to cover a whole home with one network name, handing your devices between them as you move.
Developer 2
What Is a Load Balancer?
A load balancer spreads incoming traffic across a group of servers, so no single one is overwhelmed and a failed server doesn't take the site down.
What Is localhost?
localhost is the name every computer uses for itself. Developers use it to run websites and databases locally before putting them online.