Networking
Addresses and names
A device needs an IP address, a subnet mask, a default gateway, and a DNS server. DHCP hands these out automatically; DNS resolves names to IP addresses. Mixing these up is the classic trap: if a user can ping 8.8.8.8 but not google.com, DNS is the suspect. If the device shows 169.254.x.x, DHCP failed and the host self-assigned an APIPA address.
Private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Loopback is 127.0.0.1, and a /24 mask is 255.255.255.0.
Ports worth memorizing
HTTP 80, HTTPS 443, SSH 22, Telnet 23, DNS 53, DHCP 67 and 68, SMTP 25, IMAP 143, POP3 110, RDP 3389.
Tools
ping tests reachability, traceroute or tracert shows the path, ipconfig or ifconfig shows local configuration, nslookup or dig queries DNS, and netstat shows connections. A failed ping may mean a firewall blocks ICMP, not that the host is down.
Traps
- Blaming the ISP before checking the local cable or Wi-Fi link.
- Confusing the default gateway with the DNS server.
- Assuming a link light means the network works.
- Forgetting that HTTPS uses port 443, not 80.
- Treating a private IP as reachable from the internet without NAT.
Rule of thumb
Work outward: physical link, then local IP configuration, then gateway, then DNS, then the remote service. Each step rules out a layer instead of guessing.