Allows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).

[!] --connlimit-above n
    match if the number of existing tcp connections is (not) above n

--connlimit-mask bits
    group hosts using mask

Examples:

# allow 2 telnet connections per client host
    iptables -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT

# you can also match the other way around:
    iptables -p tcp --syn --dport 23 -m connlimit ! --connlimit-above 2 -j ACCEPT

# limit the nr of parallel http requests to 16 per class C sized network (24 bit netmask)
    iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16 --connlimit-mask 24 -j REJECT