A hub (ハブ) is a concentrator that gathers cables from several devices in one place and repeats a signal received on one port out of every other port unchanged.
A port (ポート) is the socket on a device where a cable plugs in. A hub does not check where a signal is headed, so even traffic meant for one machine reaches every connected device. The more devices send at the same time, the more often signals run into each other in a collision (衝突), and efficiency drops.
AnalogyIt is like a building's PA system. Even a page for one person is heard by everyone on the floor.
How to rememberA hub is the center of a wheel. The spokes radiating from it match the way cables spread out from a hub.
Exam points
ExamWhen building a LAN, the device that brings every machine's cable together in one place is a hub (ハブ).
ExamThe physical layout with a hub in the middle and devices linked to it like rays is called a star topology (スター型).
ExamA lit link lamp (リンクランプ) only shows that the physical connection up to the port the cable is plugged into works.
A switch (スイッチ), or Layer 2 switch, is a concentrator that reads the destination MAC address of incoming data and forwards it only to the port where that device is connected.
Receive data
Record source MAC address and port in the table
Look up the destination MAC address
Forward only to the matching port
It sends the data only out that port. If the destination is not in the table yet, it sends it out every port except the one it arrived on.
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A MAC address (MACアドレス) is an ID number assigned at manufacture to each network component. A switch records each incoming frame's source MAC address and port in a table, then uses that table to work out which port leads to the destination. In this unit a plain "switch" means this Layer 2 switch, while one that decides where to send data by IP address (IPアドレス), the number assigned to devices on a network to tell them apart, is called a Layer 3 switch (レイヤ3スイッチ).
AnalogyIt is like a mailroom clerk who reads the name on each envelope and drops it only in that person's slot.
How to rememberTo switch means to change over.
Exam points
ExamDespite "switch" in its name, a Layer 3 switch (レイヤ3スイッチ) decides where to send data by the destination IP address (IPアドレス).
Mix-upA switching hub (スイッチングハブ) has "hub" (ハブ) in its name, but it is a kind of switch (スイッチ) that reads the destination and forwards accordingly.
A router (ルーター) is a device that reads the destination IP address of each packet it receives, picks a forwarding route, and relays traffic between different networks.
A packet (パケット), a small unit that data is split into for sending, carries the destination IP address. Choosing the path is called routing (ルーティング), and a router consults its routing table (経路表), which lists the next hop for each destination network, to decide where to send each packet. Routers sit on the border between separate networks, such as between office LANs or between a LAN and the internet.
AnalogyIt is like a guide at a fork in the road between towns who reads each parcel's address and sends it down the right road.
How to rememberA router is the thing that picks the route. Both words share the same root.
Exam points
ExamRouting (ルーティング) means using the destination IP address to choose the best of several paths and relay a packet (パケット) along it. With this function a router (ルーター) links different networks together.
ExamModulating and demodulating signals (converting between digital and analog) is the job of a modem (モデム), which is a separate device from a router.
ExamA wireless LAN router's guest connection feature lets visitors' devices reach only the outside and keeps them away from devices on the internal network.
A default gateway (デフォルトゲートウェイ) is the role of the exit device a host first hands a packet to when the destination lies outside its own network, and a router usually fills it.
Check the destination IP address
Decide if it is inside or outside the network
If outside, hand it to the default gateway
The router relays it to the outside network
The router that receives it picks a route and sends it out to the other network.
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Along with its own IP address and other settings, each device is configured with the IP address of its default gateway. Even without detailed route information for each destination, the device can hand outbound traffic to this exit and let it be relayed onward from there.
AnalogyMail for anywhere outside the house only needs to go in the nearest postbox. The post office carries it the rest of the way.
How to rememberDefault means the value used when nothing else is specified. It is the standard gate that traffic takes when no specific path has been set for its destination.
Exam points
ExamA default gateway (デフォルトゲートウェイ) is the doorway to other networks, and a device sends it any traffic whose destination has no specific route.
ExamWhere a router (ルーター) serves as the default gateway, each PC is set with the IP address of the router port that faces the PC's own LAN.
ExamA plain gateway (ゲートウェイ) can also mean a device that translates between networks whose communication rules (protocols, プロトコル) differ, so they cannot talk directly.
A proxy (プロキシ), or proxy server, is a server that connects to outside web servers on behalf of devices inside an office or similar network and relays the exchange.
The device sends a request to the proxy
The proxy connects to the web server instead
Return the received content to the device
The proxy passes the content back to the device that asked for it.
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Because all of the office's web traffic passes through one point, the proxy can log browsing and block connections to inappropriate sites. Outside web servers see the proxy as the party connecting, so internal devices' addresses are harder to see. It can also store pages it has already fetched and reuse them for later requests to speed up responses, which is called a cache (キャッシュ).
AnalogyIt is like a purchasing clerk who shops at outside stores for the staff. The store sees only the clerk and has a hard time telling who placed the order.
How to rememberA proxy is a stand-in, the same word as in a proxy vote.
Exam points
ExamA proxy (プロキシ) stands between the internal network and the internet, connects to web servers itself when devices ask, and passes the content back and forth. One of its main purposes is security.
ExamNAT rewrites the addresses in packets and lets them through, which differs from a proxy that makes its own new connection to the server for the device.
A VLAN (virtual LAN, 仮想LAN) is a way to split one LAN into several logical groups through switch settings, without changing the physical wiring.
One switch
Group 1 (devices on ports 1–4)
Group 2 (devices on ports 5–8)
Even on the same switch, devices in different groups are treated as separate LANs, and data does not pass directly between them. Traffic between groups has to be relayed by a router or a Layer 3 switch. When people change desks or the organization is reshuffled, group membership can be changed by settings alone, without rewiring.
How to rememberThe V stands for virtual. VPN uses the same V, so tell them apart by what follows: LAN or PN (Private Network).
Exam points
Mix-upA VLAN divides one LAN internally, which is different from a VPN, which builds a virtual private channel over the internet or another network.
SDN (Software-Defined Networking) is an approach that hands the route control of network devices over to software in one place, so the whole network's configuration can be changed centrally.
Conventional network
SDN
Forwarding decisions
Made by each device separately
Gathered into control software
Configuration changes
Set one device at a time
Pushed out together from software
Role of the devices
Both deciding and forwarding
Mainly forwarding as instructed
A network device's work can be split into control, which decides where data goes, and forwarding, which actually sends it out. SDN gathers control into software called a controller (コントローラ), and each device handles forwarding as it instructs.
AnalogyInstead of adjusting the lights at each intersection one by one, a traffic control center switches every signal in the city at once.
Exam points
Mix-upThe "software-defined" in SDN refers to how control is done, not to doing away with hardware. Switches (スイッチ) and other devices still carry the data.
Key points
A hub sends incoming signals out every other port without checking the destination, and the layout with devices radiating from a central hub is called a star topology.
A router uses the destination IP address and its routing table to choose the next hop, linking separate networks.
A device hands traffic bound outside its network to its default gateway (usually a router), and that router chooses the route from there.
A proxy connects to outside web servers for internal devices, and funneling traffic through one point makes logging and access restrictions easier.
A VLAN divides a LAN logically without rewiring, and SDN gathers network control into software so configuration changes are made all at once.