TABLE OF CONTENTS What is the correct subnet mask for the IP address 120.66.10.5/10?
255.192.0.0
255.66.0.0
255.255.10.0
255.10.0.0
A. The network ID given is a Class A network the first (w) octet is between 1 and 126, the definition of a Class A network ID. Therefore, the default subnet mask would be 255.0.0.0. You can use this as your starting point and look at the /10 after the ID. This tells you how many network bits you re using. 10 indicates you have taken two address bits from the host space to extend the default network space by two bits. The value of the left-most two bits in the second (x) octet is 128 + 64 = 192. Therefore, the new subnet mask is 255.192.0.0, and Answer A is correct.
x Answer B is incorrect because the second octet has the left-most two bits as part of the network space and the right-most six bits as part of the address space. The value 66 is obtained using the following configuration for the second ( x) octet: 01000010. This does not correspond to taking the two left-most bits as indicated by the /10 notation on the IP address and is therefore incorrect. Answer C is incorrect because this subnet mask might be used with a Class B network since the first two octets (w, x) are set to all 1s (255). In addition, if a Class A network were...
