Source NAT, or SNAT for short, is the method of changing the source address of a packet as it leaves the interface of a router. When a Neutron router is allocated an IP address from an external network, that IP is used to represent traffic that originates from virtual machine instances behind the router that do not have a floating IP. All routers in Neutron, whether they are standalone, highly-available, or distributed, support SNAT and masquerade traffic originating behind the router when floating IPs are not used.
By default, routers that handle SNAT are centralized on a single node and are not highly available, resulting in a single point of failure for a given network. As a workaround, multiple nodes may be configured in dvr_snat mode. Neutron supports the ability to leverage VRRP to provide highly-available SNAT routers, however, the feature is experimental...