Study Cisco CCST Networking addressing: IPv4, IPv6, subnet formats, public/private addresses, NAT, ARP, DNS, DHCP, and gateways.
Addressing is where many entry networking questions become practical. A support technician must recognize whether the host has a usable address, mask or prefix, gateway, DNS path, and local connectivity.
Private addresses are used inside networks and normally need NAT or routing through controlled infrastructure to reach public services. Public addresses are globally routable on the internet.
Common private IPv4 ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. On the exam, the exact arithmetic is usually less important than recognizing whether an address belongs inside a private network, whether the mask/prefix places two hosts on the same subnet, and whether off-subnet traffic has a gateway.
Treat a host network configuration as a set of dependencies:
| Field | What it proves |
|---|---|
| IP address | the endpoint has an identity on a network |
| subnet mask or prefix | the endpoint knows what is local |
| default gateway | the endpoint knows where to send off-subnet traffic |
| DNS server | the endpoint can ask for name resolution |
| MAC address | the NIC has a Layer 2 hardware identity |
If any one dependency is missing or wrong, the symptom changes. A host with no default gateway may reach local devices but fail to reach external services. A host with a wrong DNS server may ping an external IP but fail to open a site by name.
| Service | Role |
|---|---|
| ARP | maps IPv4 address to local MAC address |
| DNS | maps names to addresses |
| DHCP | assigns host network configuration |
| NAT | translates addresses, commonly private to public |
| Default gateway | next hop for off-subnet traffic |
| Cue | Likely concept |
|---|---|
| dotted decimal address such as 192.168.1.25 | IPv4 |
| colon-separated hexadecimal address | IPv6 |
| slash notation such as /24 or /64 | prefix length |
| same subnet, different host identifiers | direct local communication may be possible |
| different subnets | gateway or routing is required |
| Symptom | Better first hypothesis |
|---|---|
| No valid address after connecting | DHCP, cable, Wi-Fi association, switch port, or static configuration |
| Can reach same subnet only | default gateway or routing path |
| Can ping IP but not name | DNS |
| Can resolve name but service fails | firewall, application, port, route, or server |
| Some users affected after move | VLAN, switch port, DHCP scope, or cabling |
If a host can reach a local IP address but cannot browse a website by name, the issue may be DNS rather than cabling or gateway reachability.
Another common trap is assuming NAT fixes all addressing problems. NAT can translate private addresses for internet access, but it does not correct a wrong subnet mask, missing default gateway, broken DNS configuration, or disconnected link.