CCNA Network Infrastructure and Connectivity Practice Question
A network administrator is configuring a new Windows workstation on a small office network that uses IPv4 addressing. The workstation must be able to communicate with devices on other subnets and resolve hostnames via a company DNS server at 10.10.10.5. The administrator has already set the IP address to 10.10.10.10 and the subnet mask to 255.255.255.0. Which additional parameter must be configured to meet both requirements?
⚠ Common exam trap
Cisco often tests the requirement that a default gateway must be on the same subnet as the host; the trap here is that candidates may think a DNS server alone suffices for inter-subnet communication, or they may incorrectly assume changing the subnet mask can replace a router, or they may choose a gateway on a different subnet without realizing it is unreachable.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a default gateway of 10.10.10.1 and a DNS server of 10.10.10.5.
To communicate with devices on other subnets, the workstation needs a default gateway (router) to forward traffic beyond its local subnet. The IP address 10.10.10.10 with subnet mask 255.255.255.0 places it in the 10.10.10.0/24 network, so a default gateway (e.g., 10.10.10.1) is required for inter-subnet routing. Additionally, to resolve hostnames, the DNS server address must be explicitly configured; the company DNS server is at 10.10.10.5. Option A correctly provides both parameters.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure a default gateway of 10.10.10.1 and a DNS server of 10.10.10.5.
Why this is correct
This is the correct configuration because both entries are necessary and valid. The default gateway 10.10.10.1 lies on the same subnet as the workstation's IP address, so the host can resolve the gateway's MAC address via ARP and forward all out-of-subnet traffic to it. The router then routes packets between the 10.10.10.0/24 and 10.10.20.0/24 networks, enabling communication with devices on the other subnet. The DNS server 10.10.10.5 is also on the local subnet, allowing name resolution to work without requiring routing. Together, they provide both name resolution and the necessary Layer 3 path.
- ✗
Configure only a DNS server of 10.10.10.5.
Why it's wrong here
Configuring only a DNS server of 10.10.10.5 gives the workstation the ability to resolve hostnames into IP addresses, but name resolution does not provide any forwarding path for traffic destined outside the local subnet. The workstation's routing table has no default route, so any packet with a destination IP outside 10.10.10.0/24 is dropped as unreachable. Even though the DNS server is reachable on the same subnet, the host still cannot reach the server at 10.10.20.x because there is no gateway to route that traffic. Thus, DNS alone is insufficient for inter-subnet communication.
- ✗
Change the subnet mask to 255.255.0.0 to allow communication across subnets.
Why it's wrong here
Altering the workstation's subnet mask to 255.255.0.0 (/16) would make it assume that all 10.10.x.x addresses are within the same local broadcast domain. As a result, it would attempt to resolve destinations like 10.10.20.5 via ARP rather than forwarding them to a router, but ARP is a Layer 2 broadcast that cannot cross a router. This not only fails to provide a path to other subnets but also breaks routing from the router's perspective, since the router's interface is still configured with a /24 mask and will treat the host's /16 view as a conflicting route. The proper solution requires a Layer 3 gateway, not an expanded subnet mask.
- ✗
Configure a default gateway of 10.10.20.1 and a DNS server of 10.10.10.5.
Why it's wrong here
This configuration fails because a default gateway must be directly reachable on the same IP subnet as the host. The gateway 10.10.20.1 is on the 10.10.20.0/24 network, while the workstation is on 10.10.10.0/24, so the host cannot ARP for it and will not be able to send any frames to that gateway. Even if the DNS server 10.10.10.5 is correct, the workstation will still be unable to communicate with any remote subnet because its default route points to an unreachable next hop. The host essentially has no valid gateway, so all non-local traffic is discarded.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Configure a default gateway of 10.10.10.1 and a DNS server of 10.10.10.5.Correct answer▾
Why this is correct
This is the correct configuration because both entries are necessary and valid. The default gateway 10.10.10.1 lies on the same subnet as the workstation's IP address, so the host can resolve the gateway's MAC address via ARP and forward all out-of-subnet traffic to it. The router then routes packets between the 10.10.10.0/24 and 10.10.20.0/24 networks, enabling communication with devices on the other subnet. The DNS server 10.10.10.5 is also on the local subnet, allowing name resolution to work without requiring routing. Together, they provide both name resolution and the necessary Layer 3 path.
✗Configure only a DNS server of 10.10.10.5.Wrong answer — click to see why▾
Why this is wrong here
Without a default gateway, the workstation cannot send packets to destinations outside its own subnet (10.10.10.0/24). The DNS server alone only provides name resolution, not routing to other subnets.
Why candidates choose this
Students may think that DNS is the only missing piece because the IP and subnet mask are already set, overlooking the need for a default gateway to reach other subnets.
✗Change the subnet mask to 255.255.0.0 to allow communication across subnets.Wrong answer — click to see why▾
Why this is wrong here
Changing the subnet mask to 255.255.0.0 would expand the broadcast domain and could cause routing problems, but it does not provide a path to other subnets. The workstation still needs a default gateway to communicate with devices outside its local network.
Why candidates choose this
Some students might incorrectly believe that a larger subnet mask allows communication across subnets by including them in the same network, but this is not how routing works; it would actually break connectivity.
✗Configure a default gateway of 10.10.20.1 and a DNS server of 10.10.10.5.Wrong answer — click to see why▾
Why this is wrong here
The default gateway 10.10.20.1 is not on the same subnet as the workstation (10.10.10.0/24). For a host to reach its default gateway, the gateway must be directly reachable on the local subnet. Since 10.10.20.1 is on a different subnet, the workstation cannot send traffic to it.
Why candidates choose this
Students might think any IP can serve as a default gateway, but they must ensure it is on the same subnet. The DNS server is correct, but the gateway is misconfigured.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
Go deeper
Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
IPv4
IPv4 is the fourth version of the Internet Protocol, a set of rules that assigns unique numerical addresses to devices so they can communicate over networks like the internet.
Key term
Default gateway
A default gateway is a network device, typically a router, that acts as the exit point for traffic from a local network to other networks, including the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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