Source NAT is usually what are used.

Example of a Source NAT using a pool as a public ip address (clients on the inside have addresses from the 10.0.0.0/23-net and will be NATed to the 193.10.55.17-address when going the the internet):

#> show configuration security nat
source {
pool office-outside {
address {
193.10.55.17/32;
}
}
address-persistent;
rule-set office {
from zone office;
to zone VR-INET;
rule r1 {
match {
source-address 10.0.0.0/23;
destination-address 0.0.0.0/0;
}
then {
source-nat {
pool {
office-outside;
}
}
}
}
}
}