The correct iptables rule to allow SSH from a specific subnet is `iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT`. This rule works by appending a filter to the INPUT chain that matches TCP traffic destined for port 22, but only if the source IP falls within the 192.168.1.0/24 range, and then explicitly accepts that traffic. On the CompTIA Linux+ XK0-005 exam, this question tests your understanding of stateful packet filtering and the critical importance of the `-s` flag for source restriction—a common trap is omitting `-s`, which would open SSH to all sources, or confusing the INPUT chain with FORWARD or OUTPUT. Remember that SSH uses TCP port 22, and the subnet mask must be in CIDR notation. A helpful memory tip: "Source, Service, Accept" — always check the source (`-s`), the service port (`--dport 22`), and the target action (`-j ACCEPT`) in that order.
XK0-005 System Management Practice Question
This XK0-005 practice question tests your understanding of system management. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A system administrator is configuring a firewall using iptables. The requirement is to allow incoming SSH connections from the 192.168.1.0/24 network only. Which iptables rule should be added to the INPUT chain?
Option A is correct because it specifies the source IP range, destination port 22, and accepts the connection. Option B is wrong because it refers to the output interface. Option C is wrong because it uses REJECT instead of DROP, but more importantly, it doesn't specify source. Option D is wrong because it only logs and does not accept. Option E is wrong because -s is missing, so it accepts from any source.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related XK0-005 NAT questions on configuration and troubleshooting.
System Management — This question tests System Management — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT — Option A is correct because it specifies the source IP range, destination port 22, and accepts the connection. Option B is wrong because it refers to the output interface. Option C is wrong because it uses REJECT instead of DROP, but more importantly, it doesn't specify source. Option D is wrong because it only logs and does not accept. Option E is wrong because -s is missing, so it accepts from any source.
What should I do if I get this XK0-005 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related XK0-005 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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