关于对称加密和非对称加密的说法，正确的是(15)。

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Packet switching is a method of slicing(71) messages into parcels called "packets," sending the packets along different communication(72) as they become available, and then reassembling the packets(73) they arrive at their destination. Prior to the development of packet switching, early computer networks used leased, dedicated telephone circuits to(74) with terminals and other computers. In circuit-switched networks such as the telephone system, a complete point-to-point(75) is put together, and then communication can proceed.

URL consists of three parts: the protocol(75), the DNS name of the host (www.csai.cn) and the file name (index.html).

The essence of(74) is the invention of a hierarchical, domain-based naming scheme. It is primarily used for mapping host names and e-mail destinations to IP addresses.

As one of Gigabit Ethernet, 1000Base-Tuses(73) pairs of UTP.

To reduce their design complexity, most networks are organized as a stack of(72), each one built upon the one below it.

(71) are privately-owned networks within a single building or campus of up to a few kilometers in size.