Hello Peiwai,

a valid host address means no broadcast address and no subnet address can be used and means that not all 9 bits of host portion can be all 1 or all 0.

2) 2.172.55.33.255

you need to write in binary the last two octets

  33.255 = 001000||1 1111 1111

as you can see all last 9 bits are set to 1 so this is a broadcast address of subnet 172.55.32.0/23

6) 128.107.62.0

    62.0 in binary  =  0011 111||0  0000 0000

here all last 9 bits are set to zero this is a subnet address 128.107.62.0/23

question five is like 2)

Hope to help

Giuseppe