CCNA Network Services and Security Practice Question
A router is performing PAT for inside users. Which detail allows multiple inside sessions to share one public IPv4 address at the same time?
⚠ Common exam trap
Remember, PAT uses port numbers to differentiate sessions, not MAC addresses, IP addresses, or VLAN IDs.
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
✓
Use of transport-layer port numbers to distinguish sessions
PAT works because it uses transport-layer port numbers to keep different conversations distinct even when they share the same public IP address. In plain language, the router rewrites and tracks port information so that return traffic can be matched back to the correct inside host and application session. That is what makes one public address usable for many simultaneous internal users. This is a key difference between PAT and simple static NAT. Static NAT creates a fixed one-to-one relationship, while PAT creates many simultaneous translations differentiated by port values. The correct answer is the one that identifies port-based tracking as the reason the design scales beyond a single host.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use of transport-layer port numbers to distinguish sessions
Why this is correct
PAT (Port Address Translation) relies on the TCP or UDP port numbers to differentiate multiple simultaneous sessions from different inside hosts that all share the same public IP address. When a host sends a packet, the router assigns a unique source port, stores the mapping in its NAT table, and later uses that port to reverse the translation and deliver the response to the correct private host.
- ✗
Replacement of all MAC addresses with broadcast addresses
Why it's wrong here
Replacing all MAC addresses with broadcast addresses (FF:FF:FF:FF:FF:FF) would make every frame flood to every device on the LAN, breaking unicast delivery and overwhelming the network. PAT does not operate at Layer 2; it only modifies IP addresses and transport-layer port numbers, while MAC addresses are rewritten only for the next hop as a normal switching function.
When this WOULD be correct
In a different context, a question could ask about the effects of broadcast traffic on a network, where replacing MAC addresses with broadcast addresses might be a method to ensure that all devices receive a message, making this option correct in that scenario.
- ✗
Automatic conversion of every subnet into a /32
Why it's wrong here
Changing every subnet to a /32 mask would imply that each network contains exactly one host, which defeats the purpose of IP addressing and would isolate every device, preventing normal subnet routing. PAT does not adjust subnet masks; it works on individual translations between private and public IP addresses while leaving the inside network's mask configuration unchanged.
When this WOULD be correct
In a different context, a question could ask about the implications of subnetting in a network design where each host must be uniquely identifiable, and the correct answer would be that converting every subnet into a /32 ensures that each device has a unique address, suitable for certain routing scenarios.
- ✗
Requirement that every inside host use the same private IP address
Why it's wrong here
PAT would fail if every inside host had the same private IP address because the router could not distinguish which host originated a given session, even with unique port numbers, since the source IP would be identical. One of PAT's core requirements is that each inside host has a unique private address; the public IP is shared, but the private identities must be distinct for correct translation and return-path routing.
When this WOULD be correct
In a different scenario where the question asks about a network configuration that enforces strict IP address management, such as a highly controlled environment where multiple devices must share a single private IP for security reasons, this option could be correct. The question might specify that all devices are configured to use the same private IP for internal routing.
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.
✓Use of transport-layer port numbers to distinguish sessionsCorrect answer▾
Why this is correct
PAT (Port Address Translation) relies on the TCP or UDP port numbers to differentiate multiple simultaneous sessions from different inside hosts that all share the same public IP address. When a host sends a packet, the router assigns a unique source port, stores the mapping in its NAT table, and later uses that port to reverse the translation and deliver the response to the correct private host.
✗Replacement of all MAC addresses with broadcast addressesWrong answer — click to see why▾
Why this is wrong here
PAT does not replace MAC addresses with broadcast addresses; MAC addresses are used for local network segment communication and are not involved in NAT/PAT operations. Broadcast addresses are used for sending packets to all hosts on a subnet, which would not help in distinguishing individual sessions.
★ When this WOULD be the correct answer
In a different context, a question could ask about the effects of broadcast traffic on a network, where replacing MAC addresses with broadcast addresses might be a method to ensure that all devices receive a message, making this option correct in that scenario.
Why candidates choose this
Students might confuse the concept of address translation with MAC address manipulation or think that broadcasting is used to reach all hosts, but PAT operates at Layer 3 and Layer 4, not Layer 2.
✗Automatic conversion of every subnet into a /32Wrong answer — click to see why▾
Why this is wrong here
PAT does not convert subnets into /32 addresses; it translates private IP addresses and port numbers to a single public IP address with different port numbers. Changing subnet masks to /32 would imply host-specific routing, which is not how PAT functions.
★ When this WOULD be the correct answer
In a different context, a question could ask about the implications of subnetting in a network design where each host must be uniquely identifiable, and the correct answer would be that converting every subnet into a /32 ensures that each device has a unique address, suitable for certain routing scenarios.
Why candidates choose this
The idea of using a /32 mask might be tempting because PAT effectively makes the public IP address appear as a single host address, but the mechanism is port-based translation, not subnet mask manipulation.
✗Requirement that every inside host use the same private IP addressWrong answer — click to see why▾
Why this is wrong here
PAT does not require all inside hosts to use the same private IP address; in fact, each host typically has a unique private IP address. PAT translates these unique private addresses to the same public IP but with different port numbers to maintain session uniqueness.
★ When this WOULD be the correct answer
In a different scenario where the question asks about a network configuration that enforces strict IP address management, such as a highly controlled environment where multiple devices must share a single private IP for security reasons, this option could be correct. The question might specify that all devices are configured to use the same private IP for internal routing.
Why candidates choose this
A student might think that since PAT uses one public IP, all inside hosts must have the same private IP, but that would cause address conflicts. PAT relies on unique private addresses combined with port numbers for translation.
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?”
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Key term
IP address
An IP address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.
Key term
Port Address Translation
A networking technique that maps multiple private IP addresses and their ports to a single public IP address using unique port numbers.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.