ACSP Network Configuration 2 — Questions and Answers
Question 1: What is the purpose of DNS (Domain Name System) in macOS network configuration?
- DNS translates human-readable domain names (like apple.com) into numeric IP addresses that computers use to communicate (Correct answer)
- DNS encrypts network traffic between the Mac and internet servers for security
- DNS assigns IP addresses to devices on the local network dynamically
- DNS monitors network traffic and blocks access to malicious websites
Correct answer: DNS translates human-readable domain names (like apple.com) into numeric IP addresses that computers use to communicate
DNS resolves domain names to IP addresses. When a Mac connects to apple.com, it first queries a DNS server to get the IP address (e.g., 17.253.144.10), then connects to that IP. Without DNS, users would need to memorize numeric IP addresses.
macOS DNS configuration is set in System Settings > Network > [Interface] > Details > DNS. You can add DNS servers (Google: 8.8.8.8/8.8.4.4; Cloudflare: 1.1.1.1; your router). The 'scutil --dns' command shows the current DNS configuration including search domains. 'dscacheutil -q host -a name apple.com' performs a DNS lookup. DNS cache can be flushed with 'sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder'. Slow DNS resolution causes slow page loads even on fast connections — switching from ISP default DNS to 1.1.1.1 or 8.8.8.8 often improves performance.
Question 2: What is a DHCP reservation (also called a static DHCP or DHCP binding) and why is it useful?
- A DHCP reservation assigns a specific IP address to a device based on its MAC address, ensuring the device always gets the same IP while still using DHCP (Correct answer)
- A DHCP reservation reserves a block of IP addresses that cannot be assigned to any device on the network
- A DHCP reservation is a backup DHCP server that activates if the primary server fails
- A DHCP reservation extends the DHCP lease time for a specific device to prevent IP address expiration
Correct answer: A DHCP reservation assigns a specific IP address to a device based on its MAC address, ensuring the device always gets the same IP while still using DHCP
A DHCP reservation (static lease) tells the DHCP server to always assign the same IP address to a specific device, identified by its MAC (hardware) address. This gives a device a predictable IP without manually configuring static IP settings on the device itself.
DHCP reservations are configured on the router/DHCP server, not on the Mac. The router's MAC address binding table maps each device's hardware (MAC) address to a specific IP. This is ideal for network printers, servers, or any device that needs a consistent IP for port forwarding rules or DNS entries. On the Mac side, DHCP is still selected in Network settings — the reservation is transparent to the device. To find a Mac's MAC address: System Settings > Network > [Interface] > Details, or 'ifconfig en0 | grep ether' in Terminal.
Question 3: What is the difference between 2.4GHz and 5GHz Wi-Fi bands in terms of macOS network performance?
- 5GHz offers higher speeds and less interference but shorter range; 2.4GHz offers longer range but slower speeds and more interference from other devices (Correct answer)
- 2.4GHz and 5GHz perform identically; the difference is only in power consumption
- 5GHz works only outdoors; 2.4GHz is required for indoor use
- 2.4GHz is required for Apple devices; 5GHz is for Windows and Android only
Correct answer: 5GHz offers higher speeds and less interference but shorter range; 2.4GHz offers longer range but slower speeds and more interference from other devices
5GHz Wi-Fi provides faster speeds (up to Wi-Fi 6 and 6E standards) and less interference (fewer competing devices use 5GHz) but has shorter range and worse penetration through walls. 2.4GHz travels farther through obstacles but is crowded and slower.
2.4GHz: 3 non-overlapping channels, range up to ~100m, max speed ~600Mbps (802.11n). Microwave ovens, Bluetooth, and many IoT devices compete on 2.4GHz causing congestion. 5GHz: 23+ non-overlapping channels, range ~50m indoors, max speed 1-9.6Gbps (Wi-Fi 5/6). Wi-Fi 6E adds 6GHz band for even less congestion. macOS automatically selects the best band when connected to a dual-band router with the same SSID. For Macs used close to the router for video work or large file transfers, prefer 5GHz. For IoT devices and devices far from the router, 2.4GHz is more reliable. Option-click the Wi-Fi menu bar icon to see the current channel and band.
Question 4: How do you configure a Mac to use a specific network interface first when multiple network connections are available?
- In System Settings > Network, use the three-dot menu to set the service order, placing the preferred interface at the top (Correct answer)
- Set the preferred interface as 'Primary' in the network interface's Advanced settings
- Use the 'networksetup -ordernetworkservices' command to set priority
- macOS automatically selects the fastest available interface and cannot be manually configured
Correct answer: In System Settings > Network, use the three-dot menu to set the service order, placing the preferred interface at the top
Network service order in macOS determines which interface is used first when multiple connections are active. In System Settings > Network, click the three-dot '...' menu button and choose 'Set Service Order' to drag interfaces into priority order.
macOS uses the highest-priority interface with an active connection. Drag interfaces in the service order (Ethernet first for wired-when-possible, then Wi-Fi, then VPN) to control routing. The command-line equivalent: 'networksetup -ordernetworkservices "Ethernet" "Wi-Fi" "Bluetooth PAN"'. Check current order with 'networksetup -listnetworkserviceorder'. Note that VPN services typically install at the top automatically to route all traffic through the VPN. In corporate environments, Ethernet should usually be first for stability and security, with Wi-Fi as fallback.
Question 5: What is mDNS (Multicast DNS) and how does it enable local network device discovery on macOS?
- mDNS allows devices on the local network to announce and discover services without a central DNS server, using the .local domain suffix (e.g., MacBook.local) (Correct answer)
- mDNS is Apple's proprietary protocol for sharing files between Macs on the same network
- mDNS is a security feature that prevents DNS spoofing on local networks
- mDNS is a method of broadcasting DNS queries to all devices on the internet simultaneously
Correct answer: mDNS allows devices on the local network to announce and discover services without a central DNS server, using the .local domain suffix (e.g., MacBook.local)
mDNS (Bonjour) enables zero-configuration networking on the local network. Devices announce their services using the .local domain, and other devices can discover them without needing a DNS server — enabling AirPrint, AirPlay, and local network service discovery.
mDNS is implemented in macOS by the mDNSResponder daemon (part of Apple's Bonjour open standard). When you ping MacBook.local or access a network printer that automatically appears in Print dialog, mDNS is at work. It uses IP multicast address 224.0.0.251 (IPv4) and port 5353. DNS-SD (Service Discovery) extends mDNS to advertise services. Bonjour powers: AirPrint, AirPlay, Shared Macs in Finder sidebar, Back to My Mac, and Apple Remote Desktop discovery. 'dns-sd -B _tcp local' in Terminal browses all TCP services on the local network via DNS-SD/mDNS.
Question 6: A Mac user reports they can access websites by IP address but not by domain name. What is the most likely cause?
- DNS resolution is failing — the configured DNS servers are unreachable or returning errors (Correct answer)
- The Mac's firewall is blocking web browser traffic
- The DHCP lease has expired and the Mac has a self-assigned IP address
- The Mac's hosts file is overriding all domain lookups with incorrect entries
Correct answer: DNS resolution is failing — the configured DNS servers are unreachable or returning errors
If a Mac can reach servers by direct IP address but not by domain name, DNS lookup is failing. This isolates the problem to DNS: either the DNS servers are down/unreachable, the network is blocking port 53, or DNS settings are misconfigured.
Diagnostic steps: 1) 'nslookup apple.com' — if this fails, DNS is the issue. 2) Check DNS servers: 'scutil --dns | grep nameserver'. 3) Try an alternate DNS: 'nslookup apple.com 8.8.8.8' — if this works, the issue is with the current DNS server. 4) Flush DNS cache: 'sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder'. 5) Check /etc/hosts: 'cat /etc/hosts' for any incorrect entries. 6) Ping the DNS server IP directly to verify connectivity. Common fixes: change DNS to 8.8.8.8 or 1.1.1.1, renew DHCP lease, or check router/firewall DNS settings.
ACSP Network Configuration 2
What is the purpose of DNS (Domain Name System) in macOS network configuration?