|10.0.0.0 – 10.255.255.255 is Not THE Entire Class A block, it's a small portion of the Class A address range.|
Here's an excerpt from page 259 of TCP/IP Guide by Charles M. Kozierok, that might help.
1. If the first bit is a 0, it's a class A address, and you're done. (Half the address space has a 0 for the first bit, so this is why Class A takes up half the address space.) If it's a 1, continue to step 2.
2. If the second bit is a 0, it's a Class B address, and your done. (Half of the remaining non-Class A addresses, or one quarter of the total.) If it's a 1, continue to step 3.
3. if the third bit is a 0, then it's a Class C address and you're done. (Half again of what's left, or one-eighth of the total.) If it's a 1, continue to step 4.
4. If the fourth bit is a 0, it's a Class D address. (Half the remainder, or one-sixteenth of the address space.) if it's a 1, it's a Class E address. (The other half, one-sixteenth.)
Lowest Binary value of 1st octet: 00000001
Highest Binary value of 1st octet: 01111110
Decimal Range: 1 to 126
Address Range: 184.108.40.206 to 220.127.116.11
Note, the Class A networks 0 and 127 are reserved for special purposes.