The IP addresses on the internet are classified into five groups - A, B, C, D & E - for accommodating different network sizes. Below is the classification of class C IP as per Microsoft Technet site:

Class C addresses are used for small networks. The three high-order bits in a class C address are always set to binary 1 1 0. The next 21 bits (completing the first three octets) complete the network ID. The remaining 8 bits (last octet) represent the host ID. This allows for 2,097,152 networks and 254 hosts per network. 

Here is bulk class C IP checker tool to check up to 40 domains at time. Any harmful sites hosted on the same IP range of your site may harm the SEO of your site as well. Hence, you can view the IP range of the sites you want to check and find whether they can have any impact on your SEO.