1.5 - NAT and traffic services
Network+ N10-009 objective 1.5 covers Network Address Translation and related traffic services. Static NAT maps one private IP permanently to one public IP, useful for a server that must always be reachable at the same address. Dynamic NAT maps a pool of public IPs to private hosts on a first-come basis, while PAT (overload) shares one public IP across many hosts using ports. In NAT terminology, the private address of a host before translation is the inside local address, with inside global, outside local and outside global describing the other views. For voice, SIP sets up a VoIP call and RTP carries the actual voice media afterward. Expect scenario questions that describe an address-translation behaviour or a VoIP component and ask which NAT type, NAT term or protocol is correct.
One private to one public, permanent = static NAT. Pool of publics, first-come = dynamic NAT. Many hosts share one public via ports = PAT. Private address before translation = inside local. VoIP media = RTP (SIP sets it up).
Practice questions
1. Which transport protocol is connectionless and does not guarantee delivery?
- TCP
- UDP (correct answer)
- SCTP
- ICMP
UDP is connectionless: no handshake, no acknowledgements, lower overhead - good for DNS, VoIP, streaming. TCP is connection-oriented and reliable.
2. Which technique lets many hosts share one public IP using different port numbers?
- PAT (correct answer)
- Static NAT
- DNS round-robin
- Proxy ARP
PAT (NAT overload) maps many private IPs to one public IP by tracking unique source ports. Static NAT is one-to-one; DNS round-robin balances by rotating records.
3. Which TCP flag sequence begins the three-way handshake?
- ACK, then SYN-ACK, then FIN
- SYN, then SYN-ACK, then ACK (correct answer)
- FIN, then FIN-ACK, then RST
- RST, then SYN, then SYN-ACK
TCP opens with SYN, SYN-ACK, then ACK to establish a session. FIN closes gracefully, RST aborts abruptly.
4. Which NAT variant maps one private IP permanently to one public IP?
- PAT (overload)
- Static NAT (correct answer)
- Dynamic NAT pool
- Proxy ARP
Static NAT keeps a fixed one-to-one mapping, useful for servers that must be reachable at a stable public IP. PAT is many-to-one via ports; dynamic NAT uses a pool.
5. When a source segment is larger than a link's MTU, which value is adjusted for TCP?
- Window size
- MSS (correct answer)
- TTL
- Checksum
TCP negotiates the MSS (Maximum Segment Size) so payloads fit inside the path MTU without fragmentation. Window size controls flow, TTL limits hops.
6. In NAT terminology, what is the private address of a host before translation called?
- Inside global
- Outside local
- Outside global
- Inside local (correct answer)
The inside local address is the private IP of an internal host before NAT. Inside global is its public-facing translated address; outside addresses refer to remote hosts.