Hostapd

Scope of this document
Hostapd can do a lot of things, but only its most basic aspects will be covered here.

Requirement
A WiFi card with AP mode support is needed

WiFi Technology
A brief reminder of the technology involved

Access Point

 * An AP is like a wireless switch
 * An AP can only use one band at a time: 2.4GHz OR 5GHz, a so-called "dual-band AP" is just one AP at 2.4GHz plus one at 5GHz
 * An AP using the 2.4GHz band can be b, g and n at the same time (if the hardware supports it)
 * An AP using the 5GHz band can be a, n and ac at the same time (if the hardware supports it)
 * An AP can have multiple SSIDs, making it look like multiple APs, but all will share the same band AND channel

What it can do

 * Create an AP
 * Create multiple APs on the same card (if the card supports it, usually up to 8)
 * Create one AP on one card and another AP on a second card, all within a single instance of Hostapd

What it cannot do

 * Use 2.4GHz and 5GHz at the same time on the same card. Even if a card is dual-band it can't use both at the same time
 * Create multiple APs on different channels on the same card. Multiple APs on the same card will share the same channel
 * Create a dual-band AP, even with two cards. But it can create two APs with the same SSID
 * Assign IPs to the devices connecting to the AP, a dhcp server is needed for that
 * Assign an IP to the AP itself, it is not hostapd's job to do that

IP, DHCP and Routing
Hostapd only creates wireless ethernet switchs, it does not even know about the IP protocol or routing

IP of the AP
An AP's interface really is just an ethernet interface

DHCP
A DHCP server listening on the AP's interface will provide the AP's clients with IPs

Routing
Nothing special about routing an AP, it behaves exactly like an ethernet interface

802.11b/g/n with WPA2-PSK and CCMP
A simple but secure AP with maximal compatibility for current hardware

802.11a/n/ac with WPA2-PSK and CCMP
A simple but secure AP for recent hardware

802.11b/g/n triple AP
Three APs on the same card, one with WPA2, one with WPA1, one without encryption Hostapd automaticaly creates new interfaces for the extra APs

Dynamic Frequency Control (DFS) for 802.11ac
802.11ac works on the 5Ghz band but so do radars, in most countries this means DFS is mandatory to avoid jamming Unfortunately this is currently only supported by the following linux drivers:
 * ath5k
 * ath9k
 * ath10k

DFS explained on Wikipedia Status of DFS support in linux drivers