Consider following configuration (Solaris 11):
root@sol112:~# ifconfig -a
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
net0: flags=100001000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 2
inet 192.168.24.33 netmask ffffff00 broadcast 192.168.24.255
ether 8:0:27:75:c9:5d
net1: flags=100001000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.16 netmask ffffff00 broadcast 192.168.100.255
ether 8:0:27:57:a0:4f
lo0: flags=2002000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv6,VIRTUAL> mtu 8252 index 1
inet6 ::1/128
net0: flags=120002004841<UP,RUNNING,MULTICAST,DHCP,IPv6,PHYSRUNNING> mtu 1500 index 2
inet6 fe80::a00:27ff:fe75:c95d/10
ether 8:0:27:75:c9:5d
Let's add some IP addresses to net1:
root@sol112:~# ifconfig net1 addif 192.168.100.17/24
Created new logical interface net1:1
root@sol112:~# ifconfig net1 addif 192.168.100.17/24
Created new logical interface net1:2
After that they will be shown in ifconfig output:
# ifconfig -a
...
net1: flags=100001000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.16 netmask ffffff00 broadcast 192.168.100.255
ether 8:0:27:57:a0:4f
net1:1: flags=100001000842<BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.17 netmask ffffff00 broadcast 192.168.100.255
net1:2: flags=100001000842<BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.17 netmask ffffff00 broadcast 192.168.100.255
...
They can be easily deleted with ifconfig:
root@sol112:~# ifconfig net1 removeif 192.168.100.17
root@sol112:~# ifconfig net1 removeif 192.168.100.17
But what if we add same addresses as net1 already have?
root@sol112:~# ifconfig net1 addif 192.168.100.17/24
Created new logical interface net1:1
root@sol112:~# ifconfig net1 addif 192.168.100.16/24
Created new logical interface net1:2
root@sol112:~# ifconfig net1 addif 192.168.100.16/24
Created new logical interface net1:3
root@sol112:~# ifconfig -a
...
net1: flags=100001000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.16 netmask ffffff00 broadcast 192.168.100.255
ether 8:0:27:57:a0:4f
net1:1: flags=100001000842<BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.17 netmask ffffff00 broadcast 192.168.100.255
net1:2: flags=100001000842<BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.16 netmask ffffff00 broadcast 192.168.100.255
net1:3: flags=100001000842<BROADCAST,RUNNING,MULTICAST,IPv4,PHYSRUNNING> mtu 1500 index 3
inet 192.168.100.16 netmask ffffff00 broadcast 192.168.100.255
...
However, you cannot call up on them:
# ifconfig net1 addif 192.168.100.16/24 up
Created new logical interface net1:4
ifconfig: up: Can't assign requested address
Neither delete with ifconfig:
root@sol112:~# ifconfig net1 removeif 192.168.100.16
ifconfig: removeif: cannot remove interface: net1
root@sol112:~# ifconfig net1 removeif 192.168.100.16/24
ifconfig: 192.168.100.16/24: unexpected '/'
root@sol112:~# ifconfig net1:4 removeif 192.168.100.16
ifconfig: removeif: bad physical interface name net1:4
Only ipadm will help!
root@sol112:~# ipadm show-addr
ADDROBJ TYPE STATE ADDR
lo0/v4 static ok 127.0.0.1/8
net0/v4 static ok 192.168.24.33/24
net1/v4 static ok 192.168.100.16/24
net1/v4a static down 192.168.100.16/24
net1/v4b static down 192.168.100.16/24
net1/v4c static down 192.168.100.16/24
net1/v4d static down 192.168.100.16/24
lo0/v6 static ok ::1/128
net0/v6 addrconf ok fe80::a00:27ff:fe75:c95d/10
root@sol112:~# ipadm delete-addr net1/v4a
root@sol112:~# ipadm delete-addr net1/v4b
root@sol112:~# ipadm delete-addr net1/v4c
root@sol112:~# ipadm delete-addr net1/v4d
root@sol112:~# ipadm show-addr
ADDROBJ TYPE STATE ADDR
lo0/v4 static ok 127.0.0.1/8
net0/v4 static ok 192.168.24.33/24
net1/v4 static ok 192.168.100.16/24
lo0/v6 static ok ::1/128
net0/v6 addrconf ok fe80::a00:27ff:fe75:c95d/10
In Linux ifconfig call will silently replace current IP address. I.e.:
[root@centos6 ~]# ifconfig -a
...
eth2 Link encap:Ethernet HWaddr 08:00:27:E8:B7:2D
inet addr:192.168.100.26 Bcast:192.168.100.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fee8:b72d/64 Scope:Link
....
eth2:0 Link encap:Ethernet HWaddr 08:00:27:E8:B7:2D
inet addr:192.168.100.27 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
...
[root@centos6 ~]# ifconfig eth2 add 192.168.100.26
[root@centos6 ~]# ifconfig -a
...
eth2 Link encap:Ethernet HWaddr 08:00:27:E8:B7:2D
inet addr:192.168.100.26 Bcast:192.168.100.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fee8:b72d/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
...
And ip addr will complain about duplicate address:
[root@centos6 ~]# ip addr add 192.168.100.26/24 dev eth2
RTNETLINK answers: File exists
But you may alter netmask and everything works fine!
[root@centos6 ~]# ip addr add 192.168.100.26/25 dev eth2
[root@centos6 ~]# ip addr
...
4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 08:00:27:e8:b7:2d brd ff:ff:ff:ff:ff:ff
inet 192.168.100.26/24 scope global eth2
inet 192.168.100.26/25 scope global eth2