When you calculate subnets, it is helpful to remember that:
There is a finite amount of bits with which to work (32 bits, assuming IPv4).
Bits can either represent a network or a host, but not both. So each time a bit is allocated to one, that implicitly means it has been de-allocated from the other.
The calculations are in binary, so each time you allocate an additional bit to networks, for example, you double the quantity of available networks, but cut the quantity of hosts per network in half. (If you instead allocate an additional bit to hosts per network, the opposite is true.)
Consider a traditional class B network of 155.49.0.0. Its traditional network mask is 255.255.0.0. That is, out of the 32 bits available for a network mask, 16 of those bits are representing networks and 16 are representing hosts per network. This leaves us with 65534 usable hosts per network (2^16) - 2.
An ISP will likely reallocate this network with a longer network mask (more bits allocated to networks) to get a greater quantity of blocks of networks from that range. The ISP might allocate the 155.49.0.0 range using a 21-bit mask (255.255.248.0) instead, for example. In doing so, the ISP can find a happy medium between traditional class B and class C networks.
In the specific case of a 21-bit network mask, the ISP can reallocate the 155.49.0.0 range in 32 blocks of (8) class C networks and use a single route to represent each block. How is that achieved?
If we do not subnet the 155.49.0.0 network at all, we are left with a single network containing 2^16 - 2 or 65,534 hosts.
If we now allocate another bit to the networks (17 bits) and therefore take one away from the hosts (15), the quantity of subnets doubles to 2, and the quantity of hosts per subnet is cut to 2^15 - 2 or 32,766.
Again, each time we add a bit to the network mask, we double the quantity of subnets available, but split the quantity of hosts per network in (almost) half.
If we follow the same arithmetic for our 21-bit network mask, that yields 32 subnets. It also leaves 11 bits for the hosts or 2046 (2^11 - 2) hosts per subnet.