Communicating on the network is critical to a functioning computer system.
The Raspberry Pi OS like the vast majority of modern day OSes is setup to use DHCP to obtain and IP address automatically by default.
When you don't have a DHCP service available or when you want to assign your IP address statically you can edit the /etc/dhcpd.conf file to set a static address on the Raspberry Pi.
(NOTE: These Instructions can be used to set a static IP Address on your ethernet or wireless network interface.)
With a static address you will always know what address to use when accessing the Raspberry Pi remotely via SSH or VNC.
If you don't know your Raspberry Pi's IP Address and are running headless then please refer this blog post "How to find your Raspberry Pi on the Network." for how to find your Raspberry Pi on a network.