問51
頻出度★★★
IPv6の IP アドレスの長さは何ビットか。
平成24年度 春期 ITパスポート試験 問51
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 51
Incorrectア
32 bits is the address length of IPv4, not of IPv6. IPv4 divides the address into four 8-bit parts, writes each part as a decimal number from 0 to 255, and joins them with periods (for example 192.168.0.1). That gives 8 × 4 = 32 bits and only about 4.3 billion addresses; the exhaustion of those addresses is the very reason IPv6 was created, so 32 is wrong as the answer for IPv6.
Incorrectイ
64 bits is not the total length of an IPv6 address. However, an IPv6 address is normally used as a 64-bit network prefix in the first half (the part that identifies the network down to the subnet) plus a 64-bit interface ID in the second half (the part that identifies the device), so 64 is the number for half of the address and makes an easy trap. The whole address is twice that, 128 bits.
- ウ
正解 The correct answer is ウ (128). An IPv6 (Internet Protocol version 6) IP address is 128 bits long. The earlier IPv4 uses a 32-bit address, so it can express only 2 to the 32nd power = about 4.3 billion addresses, and as Internet users and connected devices increased rapidly, the IPv4 address exhaustion problem, in which addresses run short, became a reality. Its successor standard IPv6 therefore expanded the address length to 128 bits, four times that of IPv4, securing an address space of 2 to the 128th power = about 340 undecillion (3.4 × 10 to the 38th power) addresses, which in practice will never run out. Note that when the bit length becomes four times longer, the number of addresses does not become four times larger but 2 to the 96th power times larger. The notation also changed: IPv4 divides the address into four 8-bit parts and writes each part in decimal joined by periods (for example 192.168.0.1), while IPv6 divides it into eight 16-bit blocks and writes them in hexadecimal joined by colons (for example 2001:0db8:85a3:0000:0000:8a2e:0370:7334). The structure of 4 hexadecimal digits (that is, 16 bits) per block × 8 blocks = 16 × 8 = 128 bits also confirms that the address length is 128 bits.
Incorrectエ
No Internet protocol uses a 256-bit address. It is merely 128 doubled, and it matches neither IPv4 (32 bits) nor IPv6 (128 bits). Note also that 256 is the number of values 8 bits can represent (2 to the 8th power), so it is easy to link it with the fact that each octet of an IPv4 address takes one of the 256 values from 0 to 255 and choose it by mistake.
ここがポイント
- Memorize the address lengths — IPv4 is 32 bits (dotted decimal notation, about 4.3 billion addresses) and IPv6 is 128 bits (hexadecimal colon notation, about 340 undecillion addresses). Remember them as a set with the background that IPv6 was defined as the countermeasure to IPv4 address exhaustion.
- The relation between bit length and the number of addresses — n bits can express 2 to the nth power addresses. When the bit length goes from 32 to 128, that is four times longer, the number does not become four times larger but explodes to 2 to the 96th power times larger. Also understanding the IPv6 notation structure of 4 hexadecimal digits × 8 blocks = 128 bits makes this certain.
用語