br0.netdev

# /etc/systemd/network/br0.netdev 

[NetDev]
Name=br0
Kind=bridge

br0.network

# /etc/systemd/network/br0.network

[Match]
Name=br0

[Network]
Address=10.0.3.1/24

brctl show

λ brctl show br0
bridge name	bridge id		STP enabled	interfaces
br0		8000.c27be397d24c	no		vethVER4VO

iptables

Chain PREROUTING (policy ACCEPT 6 packets, 552 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain INPUT (policy ACCEPT 6 packets, 552 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 134 packets, 8894 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain POSTROUTING (policy ACCEPT 134 packets, 8894 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 MASQUERADE  all  --  *      wlan0   10.0.3.0/24          0.0.0.0/0     

iptables.rules

# Generated by iptables-save v1.4.21 on Sun Aug 23 16:02:57 2015
*filter
:INPUT ACCEPT [587:329452]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [543:53206]
COMMIT
# Completed on Sun Aug 23 16:02:57 2015
# Generated by iptables-save v1.4.21 on Sun Aug 23 16:02:57 2015
*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 10.0.3.0/24 -o wlan0 -j MASQUERADE
COMMIT
# Completed on Sun Aug 23 16:02:57 2015

routes

~ λ sudo route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    302    0        0 wlan0
10.0.3.0        0.0.0.0         255.255.255.0   U     0      0        0 br0
192.168.1.0     0.0.0.0         255.255.255.0   U     302    0        0 wlan0