問60
頻出度★★★
インターネットでは, 通信プロトコルとして使用されてきた IPv4 以外にも IPv6 が使用され始めている。IPv6 の説明のうち, 適切なものはどれか。
平成22年度 秋期 ITパスポート試験 問60
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 60
Incorrectア
The claim that “it cannot coexist with IPv4 networks” is wrong. IPv4 and IPv6 can coexist through a dual stack (implementing both protocols on one device), tunneling (carrying IPv6 packets across an IPv4 network), and translation technologies that convert between the two. Today’s Internet is also migrating gradually while IPv4 and IPv6 exist side by side. There is no need to build a separate, independent network.
- イ
正解 The correct answer is イ. An IPv4 address is 32 bits and an IPv6 address is 128 bits, and since 128÷32=4, the statement that “the bit length is 4 times that of IPv4” is exactly right. The number of addresses that can be expressed grows from 2^32 (about 4.3 billion) to 2^128 (about 3.4×10^38), securing a supply that will practically never run out, so the feared depletion of IP addresses can be avoided. What to note here is that it is the “bit length” that is 4 times larger, not the “number of addresses”. Because 96 more bits are added, the number of addresses becomes 2^96 times larger, an increase of a completely different order of magnitude. This distinction between “bit length ×4” and “number of addresses ×2^96” is the classic trap, and it is precisely because the choice says “the bit length is 4 times” that the statement is correct.
Incorrectウ
An IP address is still a number in IPv6. An IPv6 address is a 128-bit binary number; it is merely written in hexadecimal separated by colons (for example, 2001:db8::1) so that people can read and write it easily, and the character string itself is not the address. A string that joins a host name and a domain name (an FQDN, for example www.example.co.jp) is a name meant for humans, and it is DNS that maps such a name to an IP address. This choice confuses names with addresses.
Incorrectエ
The claim that “it has no encrypted communication function” is wrong. IPv6 builds IPsec into its specification, so encryption and authentication can be performed at the IP layer (network layer) itself. Not having to leave this to upper-layer protocols is cited as one of the improvements of IPv6 over IPv4. IPsec can also be used with IPv4 as an add-on, but it is IPv6 that assumes it as a standard from the start.
ここがポイント
- IP address length: 32 bits for IPv4 and 128 bits for IPv6. The bit length is 4 times longer, but since 96 bits are added, the number of addresses becomes 2^96 times larger. Confusing “bit length ×4” with “number of addresses ×4” is a standard trap.
- The main features of IPv6 are: an expanded address space (solving the depletion problem), standard support for IPsec enabling encryption and authentication at the IP layer, stateless address autoconfiguration that sets addresses automatically without preparing a DHCP server, and coexistence with IPv4 through dual stack and tunneling.
用語