Most AIX system administrators use: smitty tcpip to add a hostname, IP address, network mask, gateway, and DNS server setting to get the virtual machines (LPAR) and AIX on a network.  And as a side benefit so we don't need to use the ghastly VTERM console anymore as ssh or the dreaded unsecure telnet (don't do it) can now be used - as the root user:

 # smitty tcpip
  ...

  Minimum Configuration & Startup
  Further Configuration
  Use DHCP for TCPIP Configuration & Startup
  IPV6 Configuration
  Quality of Service Configuration & Startup
  Configure IP Security (IPv4)
  Configure IP Security (IPv6)

Once smitty starts, you take the "Minimum Configuration & Startup" first line menu option and then select the Network interface like en0, en1 and so on.

  +--------------------------------------------------------------------------+
  |                       Available Network Interfaces                       |
  |                                                                          |
  | Move cursor to desired item and press Enter.                             |
  |                                                                          |
  |   en0      Standard Ethernet Network Interface                           |
  |   en1      Standard Ethernet Network Interface                           |
  |   et0      IEEE 802.3 Ethernet Network Interface                         |
  |   et1      IEEE 802.3 Ethernet Network Interface                         |
  |                                                                          |
  | F1=Help                 F2=Refresh              F3=Cancel                |
  | F8=Image                F10=Exit                Enter=Do                 |
  | /=Find                  n=Find Next                                      |
  +--------------------------------------------------------------------------+

That sets up the primary network for the machine and the hostname matches the DNS server name for the IP address.

* HOSTNAME                                           [orange.aixncc.uk.ibm>
* Internet ADDRESS (dotted decimal)                  [9.133.62.82]
  Network MASK (dotted decimal)                      [255.255.255.0]
* Network INTERFACE                                   en0
  NAMESERVER
           Internet ADDRESS (dotted decimal)         [9.133.62.2]
           DOMAIN Name                               [aixncc.uk.ibm.com]
  Default Gateway
       Address (dotted decimal or symbolic name)     [9.137.62.1]
       Cost                                          [0]                     #
       Do Active Dead Gateway Detection?              no                     +
  Your CABLE Type                                     N/A                    +
  START TCP/IP daemons Now                            no                     +

But lots of machines in production have many networks for various purposes like backup, admin network, user access and clustering.

So how to set up these 2nd, 3rd, 4th and so on. Network interfaces is not so obvious.

Again start with: smitty tcpip

  $ smitty tcpip
  ...
  
  Minimum Configuration & Startup
  Further Configuration
  Use DHCP for TCPIP Configuration & Startup
  IPV6 Configuration
  Quality of Service Configuration & Startup
  Configure IP Security (IPv4)
  Configure IP Security (IPv6)

Then, the second option "Further Configuration"

  Hostname
  Static Routes
  Network Interfaces
  Name Resolution
  Client Network Services
  Server Network Services
  Manage Print Server
  Select BSD style rc Configuration
  Start Configured TCPIP Daemons
  Stop TCPIP Daemons
  Authentication Configuration

Then, down two to "Network Interfaces"

  Network Interface Selection
  Network Interface Drivers

Then, "Network Interface Selection"

  List All Network Interfaces
  Add a Network Interface
  Change / Show Characteristics of a Network Interface
  Remove a Network Interface
  PVCs for IP over an ATM Network
  Configure Aliases

Then "Change / Show Characteristics of a Network Interface"

  +--------------------------------------------------------------------------+
  |                       Available Network Interfaces                       |
  |                                                                          |
  | Move cursor to desired item and press Enter.                             |
  |                                                                          |
  |   en0      Standard Ethernet Network Interface                           |
  |   en1      Standard Ethernet Network Interface                           |
  |   et0      IEEE 802.3 Ethernet Network Interface                         |
  |   et1      IEEE 802.3 Ethernet Network Interface                         |
  |                                                                          |
  | F1=Help                 F2=Refresh              F3=Cancel                |
  | F8=Image                F10=Exit                Enter=Do                 |
  | /=Find                  n=Find Next                                      |
  +--------------------------------------------------------------------------+

Then, select the Network interface like en0, en1 but making sure you DO NOT select the primary network interface that you set above.

  Network Interface Name                              en1
  INTERNET ADDRESS (dotted decimal)                  [1.1.1.2]
  Network MASK (hexadecimal or dotted decimal)       [255.255.255.0]
  Current STATE                                       up                     +
  Use Address Resolution Protocol (ARP)?              yes                    +
  BROADCAST ADDRESS (dotted decimal)                 []
  Interface Specific Network Options
    ('NULL' will unset the option)
    rfc1323                                          []
    tcp_mssdflt                                      []
    tcp_nodelay                                      []
    tcp_recvspace                                    []
    tcp_sendspace                                    []
  Apply change to DATABASE only                       no                     +

This gives you a simpler panel to fill in but NOT the hostname, gateway nor the DNS details - these are only set once and while you are setting up the primary network as they apply to the other networks too.

Using the "Minimum Configuration & Startup" panel to set up the 2nd, 3rd and so on network interfaces is a VERY BAD IDEA as it messes up the network configuration but I have seen it done many times. 

*** FLAME ON ***

Before I get lots of comments about why does smitty allows this, if it causes problems, I would like to say UNIX, AIX and smitty are not meant to be "idiot proof" and I like it the way it is. Idiots, newbies to UNIX and the untrained must not be given root access.

*** FLAME OFF ***