There are some significant differences between the default configurations of Ubuntu and CentOS. Both use the /etc/sysctl.conf file, but on CentOS, that file doesn't have anything except for some explanatory comments. Ubuntu and CentOS both have files with default settings in the /usr/lib/sysctl.d/ directory, but there are more for CentOS than there are for Ubuntu. On Ubuntu, you'll find other files with default values in the /etc/sysctl.d directory. On CentOS, that directory only contains a symbolic link that points back to the /etc/sysctl.conf file. Also, you'll find that some things are hardcoded into the Linux kernel and aren't mentioned in any of the configuration files. In true Linux fashion, every distro has a different way of configuring all this, just to ensure that users remain thoroughly confused. But that's okay. We'll try to make sense of it, anyway.