for centos 6.x it is changed to the following codes :
To print current limit type:
# sysctl net.nf_conntrack_max
Output:65536
To increase this limit to e.g. 100000, type:
# sysctl -w net.nf_conntrack_max=100000
To make this settings permanent add the following line to /etc/sysctl.conf file:
net.nf_conntrack_max = 100000
The following will tell you how many sessions are open right now:
# wc -l /proc/net/nf_conntrack