ITPEC FE Morning October 2018 Question 36
As shown below, multiple PCs are connected to the home network and they have access to the Internet through FTTH, and a global IP address is assigned to the WAN interface of device A. Which of the following is the function of device A that enables the PCs to use the Internet with this 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.
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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