Subaddress is 42(0x2a)

https://github.com/monerobook/monerobook/blob/master/chapters/5.md#5341-creating-a-subaddress

`However, the mainnet network byte for subaddresses is 0x42, which is why they all begin with the digit an '8'.`

this should be:

`However, the mainnet network byte for subaddresses is 42(0x2a), which is why they all begin with the digit an '8'.`
