ITPEC FE Morning April 2022 Question 30
Multiple PCs are connected to the Internet with FTTH. The WAN interface of device A has a single global IP address allocated to it. Which of the following is the function of device A that enables these PCs to connect to the Internet by sharing the global IP address?
Explanation
NAPT (IP masquerade) — it translates each PC's private address and port into the one global address with a distinct port, so all the PCs share it.
The PCs behind device A have private IP addresses, which cannot be routed on the Internet. When a PC sends a packet, NAPT replaces its source address and port with the global address and a port number unique to that session, and records the pair in a translation table. A reply arrives at the global address and that port, and the table tells device A which PC and port to forward it to. Because sessions are told apart by port number, many PCs can use the one address at the same time; plain NAT, which translates addresses only, would need one global address per PC communicating at once.
Answer (b)
Why not others:
- •(a) DHCP — assigns IP addresses and settings to the PCs on the LAN, but translates nothing; the private addresses it hands out still cannot reach the Internet.
- •(c) Packet filtering — passes or drops packets by address, port and direction, a firewall function that does not change addresses.
- •(d) PPPoE — sets up the authenticated session with the ISP over the FTTH line, through which the single global address is obtained; it does not let several PCs share it.
Key rule: NAT maps addresses one to one; NAPT adds port numbers, so many private hosts can share a single global address.
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