New users of OpenWrt often get into OpenWrt because they want a more advanced or customizable or secure network than what is possible with an off the shelf wifi router device running factory firmware. However, getting things set up is easier if they understand the proper usage of network terminology. Because they are often using devices that are "all in one" and sold as a router, there is a tendency to call their device "a router". In reality it's useful to have a little more nuance so that when they ask questions the more network savvy people understand what they mean. Here are some basic terminology definitions:
- All in one device: a device with a CPU, usually a built in multi-port switch, and wifi radios, optionally including a DSL modem. This is often called "a wireless router". These are the most common platform for running OpenWrt (however there are wired only devices as well).
- Router: Any computer that is connected to two or more separate IP/IPv6 networks and acts to forward packets between the two networks on the basis of the IP addresses.
- Access Point: Any device which enables wireless client devices to send packets to other devices in a network. Usually a connection between wireless devices and a wired network or between two wireless clients.
- Switch: A device which forwards ethernet frames between different physical ports based mainly on the destination MAC address. Usually offering many ports at once, such as 5,8,16,24 etc so that many devices can be plugged into a network.
- Modem: a device for converting digital data into analog signals suitable for transmitting over analog copper telephone lines (ADSL or VDSL modem) or cable television coax (DOCSIS/cable modem). OpenWrt can operate some DSL modems but DOCSIS/cable modems must always be ISP approved devices.
- ONT/Media Converter: An optical network terminator. Generally a device that acts like a modem to convert digital signals to/from ethernet and optical fiber networks.
- Managed Switch: A switch that is aware of VLANs and can be configured for advanced switching functions like Multicast/IGMP snooping, Link Aggregation, QoS, or other features
- Dumb Switch: A switch with a bunch of ports that treat all ports symmetrically.
- VLAN: A way to separate ethernet networks even though they run over the same physical cables. VLANs tag packets with a VLAN number and then VLAN aware switches treat them as if they were in different separated networks. It generally requires a router to route between these segments even though they are traveling over the same wires.
- ESSID: an Extended Service Set IDentifier (or sometimes SSID) is a "network name" like "MyHomeNetwork" which identifies access points which all are connected to the same network, and can be roamed between without breaking connections. This is generally what is shown on a wireless device when attempting to "choose a network". Physical Access Points can offer multiple ESSIDs with one radio, with different security or VLAN tags associated with those different ESSIDs.
- BSSID: the "MAC Address" of a particular single access point which may be part of a larger Extended SSID (ESSID).
When one buys an "all in one" device and configures it normally, it is simultaneously a router, a managed switch, and an access point (or two) in one piece of hardware. It is also possible to configure these all in one devices as strictly an access point (dumb AP). In this role its goal is just to move packets from the wired LAN to WiFi clients and back without considering the IP/IPv6 addresses at all, just using the MAC addresses of the packets. In this role, it doesn't normally do firewalling or decision making about whether the packets are allowed, or should be sent out one connection or another. It is in essence a "switch" in which one port is "plugged into the wired network" and one port is "plugged into the air".
As networks are getting more comprehensive, including IPTV, VOIP phones, laptops, smartphones, desktop machines, servers, and internet of things appliances, more and more people are finding it important to segment their network into separate groups of devices using VLANs and different ESSIDs for different groups of devices. In general there is a mapping between ESSID and VLAN tag so that all devices connecting to a given ESSID are also connected to a particular VLAN on the wired side.
Hopefully this post can act as a primer for those getting started in OpenWrt so that they can better ask questions, and understand the answers they get.