assigning alias ip address

[John Black]

How does assigning alias IP address work?

I have one network jack coming into my server room. 
eth0  10.10.10.10

I have two servers with public IP address.
this assigns them, right?
ifconfig eth0:10 192.168.1.10 netmask 255.255.255.0
ifconfig eth0:11 192.168.2.11 netmask 255.255.255.0

iptables -t nat -A PREROUTING -d 192.168.1.10 -i eth0 \
                -j DNAT --to-destination 10.10.10.11
iptables -t nat -A PREROUTING -d 192.168.2.11 -i eth0 \
                -j DNAT --to-destination 10.10.10.12

When a mail or web request is made from the internet.  
How does it know where the servers are?

thanks
John