問72
頻出度★★★
ブロードバンドルータなどに搭載されているNATの機能として，適切なものはどれか。
平成25年度 春期 ITパスポート試験 問72
テクノロジ系ネットワーク › ネットワーク応用
This question's explanation and your answers so far.
解説
問 72
Incorrectア
Mutually converting between a global IP address and a domain name is the role of DNS (Domain Name System), not NAT. DNS is the mechanism that maps a domain name which is easy for people to remember, such as www.example.co.jp, to the IP address needed for communication and answers with it, and the server that holds this mapping table is the DNS server. Looking up an IP address from a name is called forward lookup, and looking up a name from an IP address is called reverse lookup. A broadband router may have a function that relays DNS queries, but that is not a function of NAT.
Incorrectイ
There is no mechanism that mutually converts between a global IP address and an email address. There is indeed a flow in which the domain name after the @ in an email address is used to look up the destination mail server through a DNS MX record, and that server name is then converted into an IP address, but this is a matter of DNS and mail delivery (SMTP); it does not mean that an IP address and an email address can be converted into each other one to one. Naturally, this is not a function of NAT either.
Incorrectウ
Obtaining the correspondence between an IP address and a MAC address is the role of ARP (Address Resolution Protocol), not NAT. A MAC address is a unique physical address given to a NIC (network interface card) when it is manufactured, and it is used to deliver frames within the same LAN. ARP is a protocol that broadcasts a query inside the LAN to find the MAC address of the device that has the destination IP address. Its layer and its purpose are both different from those of NAT, and in the first place a MAC address is not something meant to be rewritten in order to communicate with the outside.
- エ
正解 The correct answer is エ. NAT stands for Network Address Translation; in Japanese it is called network address translation. Inside a home or company LAN, each device is given a private IP address (such as 192.168.x.x, a number that is valid only within the organization). To communicate on the Internet, however, a global IP address that is not duplicated anywhere in the world is required. For that reason the broadband router placed at the boundary between the LAN and the Internet rewrites the source address of packets going out from the inside, changing it from the private IP address to the global IP address that the router borrows from its ISP (Internet service provider); and for packets that come back, it does the reverse, rewriting the global IP address back into the original private IP address. This mutual conversion is exactly the function of NAT, and it matches the description in choice エ. Thanks to this mechanism, even when only one global IP address has been contracted, several PCs and smartphones in the LAN can all use the Internet, which saves the limited supply of IPv4 addresses. In addition, because the addresses of the individual devices inside the LAN cannot be seen from outside, it also has the secondary effect of improving security. Note that an actual broadband router converts port numbers as well as IP addresses so that a single global IP address can be shared by many terminals, which is called NAPT (IP masquerade); this too is an extension of NAT, and its essence, converting IP addresses, is the same.
ここがポイント
- NAT (Network Address Translation) is the function that mutually converts between private IP addresses, which are valid only inside a LAN, and global IP addresses, which are valid on the Internet. It is built into broadband routers and lets many terminals connect to the Internet with only a few global IP addresses (saving IPv4 addresses). The extension that also converts port numbers so that a single global IP address can be shared by several terminals is NAPT (IP masquerade).
- Distinguish address conversion terms by asking what is converted into what. NAT deals with private IP addresses and global IP addresses, DNS with domain names and IP addresses, ARP with IP addresses and MAC addresses, while DHCP performs automatic assignment of IP addresses (not conversion). A broadband router often provides these functions at the same time, so be careful not to confuse the name of a function with its role.
用語