200-901 network fundamentals explained
Network Fundamentals is 15% of 200-901. It is a conceptual layer: you describe and interpret, you do not configure a router. It matters because every automation script runs on a network, and the exam wants to know that you can tell whether a failure is in your code or in the path to the API. If you hold the CCNA, this domain is mostly review; if you do not, the CCNA infrastructure guide is a deeper companion.
Addressing and switching
MAC addresses are 48-bit hardware addresses used inside a local network segment. VLANs split a switch into separate broadcast domains, so devices in different VLANs need a router to communicate. See also switching and network access.
IP addresses identify hosts across networks. The subnet mask or prefix (for example /24) marks the network part. A host sends traffic to its default gateway when the destination is outside its subnet, and routes tell a router where to send each destination.
| Concept | Purpose |
|---|---|
| MAC address | Local delivery on a segment |
| VLAN | Logical separation of a switch |
| IP address | Delivery across networks |
| Mask / prefix | Splits network from host portion |
| Gateway | Exit for non-local traffic |
| Route | Next hop for a destination |
Network components and topology
| Device | Function |
|---|---|
| Switch | Forwards frames by MAC address inside a network |
| Router | Forwards packets between networks by IP address |
| Firewall | Permits or denies traffic by policy |
| Load balancer | Distributes requests across servers |
In a topology diagram, trace the path from a client through the switch, router and firewall to a server and note the ports shown. Questions often ask where a failure sits.
The three planes
| Plane | Role |
|---|---|
| Management | Configure and monitor the device (SSH, NETCONF, SNMP) |
| Control | Decide where traffic goes (routing protocols, ARP) |
| Data | Forward the traffic itself |
APIs and automation mostly use the management plane.
IP services
| Service | What it does |
|---|---|
| DHCP | Hands out addresses, masks, gateways and DNS servers |
| DNS | Resolves names to IP addresses |
| NAT | Translates private addresses to public ones |
| SNMP | Monitors and manages devices |
| NTP | Synchronises clocks, essential for logs and certificates |
Common ports
| Port | Protocol |
|---|---|
| 22 | SSH |
| 23 | Telnet |
| 53 | DNS |
| 80 | HTTP |
| 123 | NTP |
| 161 | SNMP |
| 443 | HTTPS (also RESTCONF) |
| 830 | NETCONF over SSH |
Telnet sends everything in clear text, so SSH is preferred.
Diagnosing application connectivity
When an application cannot reach an API, think layer by layer. A NAT problem shows up when return traffic cannot find the internal host. A blocked transport port means the firewall drops traffic to a port such as 443. A proxy may be required to reach external sites, and a VPN changes the route and sometimes DNS. Start with the simple checks: does the name resolve, is the address reachable, is the port open, does the credential work.
Network constraints
Latency slows every call, limited bandwidth hurts large transfers, packet loss triggers retries and jitter harms real-time media. Chatty applications that make many small calls suffer most from latency, so batching requests helps.
Example questions
Question 1. A script on a laptop can resolve api.example.com and ping the server, but the HTTPS request times out. Other users on the same network reach the site normally through a corporate proxy. What is the most likely cause?
- A. DNS cannot resolve the name
- B. The API token has expired
- C. Direct outbound HTTPS is blocked and the script needs the proxy
- D. The laptop is in the wrong VLAN number
Show answer
Answer: C
Resolution and ping working but HTTPS timing out, while others succeed through a proxy, suggests direct outbound traffic on port 443 is blocked and the script must use the proxy. A DNS failure would stop name resolution. An expired token would return 401, not a timeout. A wrong VLAN number would not allow ping to work.
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Question 2. Which service lets a new device on a network obtain an IP address, subnet mask, default gateway and DNS server automatically?
- A. DNS
- B. DHCP
- C. NAT
- D. NTP
Show answer
Answer: B
DHCP delivers those settings automatically. DNS only resolves names to addresses. NAT translates addresses at a boundary. NTP synchronises time and provides no addressing.
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Question 3. An engineer connects to a switch with NETCONF. Which transport and well-known port does it normally use?
- A. SSH, port 830
- B. Telnet, port 23
- C. HTTP, port 80
- D. SNMP, port 161
Show answer
Answer: A
NETCONF runs over SSH and uses port 830 by convention. Port 23 is Telnet. Port 80 is unencrypted HTTP. Port 161 is SNMP.
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Back to the exam format, or test yourself with the practice test. For the exam’s relation to the networking associate, see 200-901 vs CCNA.