인터넷, 웹 및 IP 주소 ⟦The internet, the web and IP addressing⟧

A network so successful it ran out of numbers

• When the internet's addressing scheme was designed in 1981, its 32-bit addresses allowed about four billion devices. That seemed inexhaustible for an experiment linking a few hundred universities.
• The last free blocks were handed out in 2011. Every phone, laptop, camera and sensor on Earth wants an address, and there were not enough to go round.
• The replacement has 128 bits: enough for every grain of sand on the planet to have billions of addresses each.
• This lesson is the internet 互联网 itself, the World Wide Web 万维网 that runs on it, and the addresses that let a packet find one device among billions.

The internet and the World Wide Web

• The internet is a global network of networks that communicate using a common protocol 协议 suite, TCP/IP.
• The World Wide Web (WWW) is one service that runs over it: hyperlinked documents identified by URLs, requested with HTTP or HTTPS and shown in a browser.
• Email and file transfer are other internet services that are not part of the web. Webmail uses both: the mailbox is a web page reached through a browser, while the email itself travels across the network of networks.

The web rides on the internet; so do services that are not the web

Hardware that supports the internet

• A modem 调制解调器 converts the computer's digital signal into an analogue signal for a telephone line, and back again at the other end: modulation and demodulation.
• The PSTN, the public telephone network of exchanges and lines, carries dial-up and DSL connections. A dedicated line 专线 is leased between an organisation and its ISP: always on, a fixed bandwidth that is not shared, faster and more reliable, but expensive.
• On the cell phone network 蜂窝网络 a phone sends data by radio to the nearest tower; the towers are linked to the phone company's network, which routes the data to the internet, and the phone is handed from cell to cell as it moves.

Three routes onto the internet

IP addresses: IPv4 and IPv6

• An IP address IP地址 uniquely identifies a device on a network, so a packet can be delivered to it.
• IPv4 第四版互联网协议 is 32 bits, written as four denary numbers from 0 to 255 separated by dots, 192.168.1.10: about $4.3 \times 10^{9}$ addresses, now exhausted.
• IPv6 第六版互联网协议 is 128 bits, written as eight groups of four hexadecimal digits separated by colons, 2001:0db8:0000:0000:0000:ff00:0042:8329; a run of zero groups shortens to ::. About $3.4 \times 10^{38}$ addresses.

Worked example: is this an IPv4 or an IPv6 address?

Subnetting

• A large network is split into subnets 子网. An address then has two parts: the network ID 网络标识, the same for every device in the subnet, and the host ID 主机标识, unique to each device.
• The subnet mask 子网掩码 marks the split: 255.255.255.0 says the first 24 bits are the network part, so up to 254 hosts share the last 8.
• Benefits: broadcasts stay inside their subnet, so there is less traffic on each part; one department's devices can be secured separately; and the network is easier to manage and to grow.

One network, several subnets, one network ID each

Worked example: reading a subnet mask

Public and private addresses

• A public IP address 公有IP地址 is globally unique and routable across the internet, allocated by the ISP 互联网服务提供商.
• A private IP address 私有IP地址 is used only inside a LAN, is not routable on the internet, and is reused by millions of LANs, 192.168.x.x for example. Devices with private addresses reach the internet through NAT 网络地址转换 in the router.
• The security implication: a device with only a private address cannot be reached directly from the internet, which is a layer of protection, while a public address is visible to the whole world.

Static and dynamic addresses

• A static IP address 静态IP地址 never changes. It is set by hand or reserved, and it suits a server that clients must find at a known address.
• A dynamic IP address 动态IP地址 is allocated by DHCP 动态主机配置协议 each time a device connects and may be different next time. It suits laptops and phones, and it uses a limited pool of addresses efficiently.
• Static costs more and needs administration; dynamic is automatic but a device's address cannot be relied on.

Worked example: the descriptions the tables want

Marks that slip away

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