What does it mean when a channel is in brackets? For example, 42 [48]

When you see two channel numbers and one is in brackets, this means that a wider-than-20-MHz channel width is being used.

In the 5 GHz band, you can have up to 40, 80, or 160 MHz wide networks. The first number denotes the center channel of that wide network, where as the channel in brackets denotes the channel being used by primary 20 MHz wide network. You can learn more about 802.11ac naming conventions here.

In the 2.4 GHz band, you will see a slightly different naming convention since 802.11ac does not operate in this band. If 40 MHz wide networks are being used in the 2.4 GHz band, you will see a + or - sign used, for example 1 + 5, or 11 - 7.

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In this “old” way of naming (802.11n) the first number is the primary 20 MHz wide channel and the second number is the secondary 20 MHz wide channel. You can learn more about how 802.11n networks are named here.