Unless your network plans to be private in perpetuity, your local users most likely need to communicate beyond the local network. Therefore, you must obtain a registered IP number for your network from the appropriate organization before your network can communicate externally. This address becomes the network number for your IPv4 addressing scheme or the site prefix for your IPv6 addressing scheme.
Internet Service Providers provide IP addresses for networks with pricing that is based on different levels of service. Investigate with various ISPs to determine which provides the best service for your network. ISP's typically offer dynamically allocated addresses or static IP addresses to businesses. Some ISPs offer both IPv4 and IPv6 addresses.
If your site is an ISP, you obtain IP address blocks for your customers from the Internet Registry (IR) for your locale. The Internet Assigned Numbers Authority (IANA) is ultimately responsible for delegating registered IP addresses to IRs around the world. Each IR has registration information and templates for the locale that the IR services. For information about the IANA and its IRs, refer to the IANA's IP Address Service page.
Do not arbitrarily assign IP addresses to your network, even if you are not currently attaching the network to external TCP/IP networks. Instead, use private addresses as described in Using Private IPv4 Addresses.