An IP address (IPアドレス) is the number that identifies the device you are communicating with on a network, and it is not the same thing as the MAC address (MACアドレス) built into a communication component.
Add the destination IP address
Router forwards to the next hop
Reaches the other device
After repeated forwarding, the packet arrives at the device that holds the destination IP address.
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Data travels in small units called packets, and a router (ルータ), the device that relays between networks, reads the destination IP address to choose the next hop. The address splits into a network portion (ネットワークアドレス部) that names the network and a host portion (ホストアドレス部) that names a device on it, with a value called the subnet mask (サブネットマスク) setting the boundary. Strings of digits are hard to remember, so people normally use a text domain name (ドメイン名), and a system called DNS looks up the matching IP address.
AnalogyThink of the delivery address on a parcel label. The street narrows it to a delivery area, and the house number picks out the home.
How to rememberIP stands for Internet Protocol, so the P means a set of communication rules, not a number.
Exam points
ExamA subnet mask (サブネットマスク) is a value that marks how much of an address indicates the network. It neither translates addresses nor assigns them automatically.
Mix-upDNS links names such as domain names (ドメイン名) and host names (ホスト名) with IP addresses (IPアドレス). Mapping to MAC addresses (MACアドレス) and handing addresses to devices automatically are the jobs of other mechanisms.
ExamWhen a DNS server (DNSサーバ) goes down, the lines and routers (ルータ) keep running. Only access by name is affected, and access by IP address still works.
IPv4 is version 4 of IP, the internet's communication protocol, and it writes an address in 32 bits (32ビット).
Notation
Group 1
Group 2
Group 3
Group 4
Binary (8 bits each)
11001011
00000000
01110001
00011001
Decimal (203.0.113.25)
203
0
113
25
It can express 2 to the 32nd power addresses, roughly 4.3 billion, and the shortage became a problem as more devices came online. To write one, you split the 32 bits into four groups of 8 bits, turn each group into a decimal number from 0 to 255, and join them with dots.
AnalogyThink of a membership number with a fixed length. Unless you add digits, the total supply of numbers stays the same however many members join.
How to rememberThe 4 in v4 is a version number, but it happens to match the number of dotted groups, so you can recall 4 groups × 8 bits = 32.
Exam points
ExamIf any group contains a letter, or the number of groups is not four, it is not an IPv4 address.
ExamIPv4 addresses have a fixed ceiling, so there are not enough to go around for every device connected directly to the internet.
IPv6 is the IP version created to end the IPv4 address shortage, and it extends the address length to 128 bits (128ビット).
It can express 2 to the 128th power addresses, a scale at which running out is no practical concern. The notation is hexadecimal in 16-bit groups separated by colons, so even when letters appear it is still a numeric value, not a name like a domain name.
AnalogyThink of making a membership number much longer. Each added digit multiplies the total supply of numbers.
Exam points
ExamAn IPv6 address is 128 bits (128ビット) long, four times the length of an IPv4 address.
ExamWhat IPv6 solves is the address shortage. It has nothing to do with faster communication or a need for a particular cable (ケーブル).
ExamIPv6 and IPv4 can coexist on one network, so there is no need to build them as separate networks.
A global IP address (グローバルIPアドレス) is an IP address assigned on the internet with no duplicate anywhere in the world, and it can be reached directly from outside.
If one number existed in two places, the destination would be ambiguous, so these addresses are assigned through providers (ISPs) and others, within a framework headed by ICANN, the international body that manages the numbers. Devices that accept outside connections, such as web servers, normally use this kind of address.
AnalogyThink of a company's main phone number. Call it from anywhere in the world and you reach the same company.
How to rememberGlobal shares its root with globe, which points to one of a kind on the whole planet.
Exam points
ExamYou cannot set a global IP address (グローバルIPアドレス) without authorization, even if it looks unused. Nor can the same value be given to several machines at once.
ExamA global IP address is a configured value, not a number burned into the hardware, so a replacement machine can take over the same value.
A private IP address (プライベートIPアドレス) is an IP address used only inside an internal network, such as at home or in a company.
Device on the internal network
Address translation (NAT)
Host on the internet
Reply translated back to the device
NAT changes the destination back to the private IP address, and the reply reaches the device on the internal network.
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You pick from reserved ranges, such as the one starting with 192.168, and assign addresses so none repeats across the internal network. A private address cannot talk to the internet as it stands, so a function called NAT (Network Address Translation) converts between it and a global IP address.
AnalogyThink of an internal extension that works only inside a company. Another company can have the same extension without trouble, and outside calls connect through the main number.
How to rememberIt is the private of private room, so picture a number that does not reach beyond the room on its own.
Exam points
ExamNAT converts between the two kinds of IP address (IPアドレス): private (プライベート) and global (グローバル). Converting between IP addresses and domain names (ドメイン名) is the job of DNS.
ExamUsing a private IP address (プライベートIPアドレス) requires no application to an ISP, and ICANN does not allocate it.
ExamThere is no limit on the kind of device it can go to, so a server (サーバ) on the internal network can use a private IP address too.