IP Aliasing



In a LAN environment, clients and servers are connected to the network by 
one or more network controller cards. Each controller has a factory-set 
(hard-coded) primary physical address called the MAC address. The MAC 
address is also known as the primary unicast address. For an Ethernet
controller, the MAC address is a string of hex bytes that looks, for  
example, like this:

        08:0a:10:bc:7f:5d

In addition to its own name, the controller can recognize up to 16 alias 
names placed in the its driver's multicast address list. The controller
accepts any packet sent to its primary unicast address and any packets 
sent to an address in its multicast list.

The IP address is a unique name given to a controller to identify it on 
the network. The IP address is a broken numeric string that looks similar 
to:

        129.226.55.33

Traditional internet protocol (IP) allows each controller on a network to 
have one IP address, which is mapped to the MAC address to establish its 
network identity. IP aliasing allows each controller to have a secondary 
IP address (an alias) supporting up to 16 unique virtual addresses, so 
that a network administrator can greatly expand the effective size of the 
network without installing additional hardware. The virtual addresses are 
mapped to the multicast list of the physical controller.

The utilities ipalias and macconfig are used to create multiple virtual 
interfaces on a single physical network interface. Although the primary 
interface is brought on line with ifconfig, the IP layer treats the 
primary and secondary addresses as equals with identical responses ping, 
ftp, telnet, rcp, rlogin, and so forth.

Aliasing the MAC Address

The macconfig utility is used to add a multicast address to the list of 
multicast addresses on system X. This task involves the conversion of a 
unicast address to a new form, the multicast address. An understanding of 
48-bit Universal LAN MAC address structure makes this task easier.

48-bit Universal LAN MAC Address

The concept of universal addressing is based on the requirement that all 
members of a network have a unique identifier. Otherwise they cannot 
coexist. The advantage of a universal address is that a node with such an 
address can be attached to any LAN in the world with an assurance that its 
address is unique.

The 48-bit Universal Address consists of two parts:

1. The first 24 bits correspond to the Organizationally Unique Identifier 
(OUI) as assigned by the IEEE, except that the assignee may set a control 
bit for group addresses (multicast addresses) or for individual addresses 
(unicast addresses).2. The second part, comprising the remaining 24 bits, 
is administered locally by the assignee. In the following 48 bit LAN MAC 
address, the Organizationally Unique Identifier is contained in octets 0, 
1, 2 and the remaining octets 3, 4 and 5 are assigned locally.

Octet   0               1               1               3               4               5
Binary  0011    0101    0111    1011    0001    0010    0000    0000    0000    0000    0000    0001
Hex     C       A       E       D       8       4       0       0       0       0       0       8

The least significant bit or first bit octet 0 is the I/G address bit used 
to identify the destination address, either as an individual/unicast 
address (0) or as a group/multicast address (1) that identifies one, 
several, or all stations connected to the LAN. The all-station broadcast 
address is a special predefined group address off all 1's.

The second bit of octet 0 is the Universally or Locally Administered (U/L) 
bit. This bit indicates whether the address has been assigned by a local or 
universal administrator. Universally administered addresses have this bit 
set to 0 and they are globally/universally unique. This is the normal 
case. If this bit is set to 1, the entire address (all 48 bits) has been 
locally administered and may not be globally unique. Thus, for example, 
bytes 0,1 and 2 would be unique for SynnerGy Networks, and 
SynnerGy Networks can then assign the last 24 bits for each network 
interface controller it manufactures, thereby allowing approximately 16 
million individual addresses and 16 million unique group addresses that 
no other organization can have (universally unique).

Converting a Unicast Address to a Multicast Address

For a given 48-bit Universal LAN MAC address, the first 24 bits 
correspond to OUI. The least significant bit (the first bit or I/G bit) 
of the OUI is the only bit that can be changed to make this address a 
multicast address. We cannot change any other bit without either 
compromising uniqueness of the address or violating some IEEE addressing 
standards.

The U/L bit for a universally unique address is always 0. This leaves the 
following four possibilities in the least significant nibble (4 bits) of 
a valid universally unique unicast/individual address:

0x0(0000)
0x4(0100)
0x8(1000)
0xC(1100)

where 0x indicates a hexadecimal digit.

Given the fact that we can only change one bit (the I/G bit) to convert 
the unicast address to a multicast address, we have the following 
transformations:

0x0(0000) ? 0x1(0001)
0x4(0100) ? 0x5(0101)
0x8(1000) ? 0x9(1001)
0xC(1100) ? 0xD(1101)

Thus, for a given unicast address, 08:00:0b:06:1d:50, for example, 
the multicast form is 
09:00:0b:06:1d:50.


Address Resolution Protocol

The IP addresses must be mapped to MAC addresses before a client can 
successfully send a packet to a server or peer on the network. In TCP/IP, 
the Address Resolution Protocol (ARP) is used to seek and map the address 
information. The client sends an ARP broadcast query that says, in effect,
"Here is a destination IP address. Send me your MAC address." This query 
is seen by all the controllers on the network, and the owner of the IP 
address replies with its MAC address. The client stores the IP/MAC address
map entry in a small  table called the ARP cache. This information can be
published by ARP.

Aliasing the IP address - An Example

Initially, the network controller is brought online and given a primary IP 
address using ifconfig. Now we use ipalias to create up to 16 additional 
network identities on the secondary IP address and will honor all 
communication sent to its IP Aliases.

Steps to bring a virtual network interface up on a system are:

1. Add the alias IP address to the interface using the following command 
   sequence:

$ ipalias -a 129.226.55.33 et5961

where (as an example) 129.226.55.33 is the alias IP address and et5961 is 
the controller name.

2. Add the multicast address to the interface

$ macconfig -m 129.226.55.33 9:6:a:b:3:e et5961

where 9:6:a:b:3:e is the example MAC address and et5961 is the controller 
name.

3. Invoke ARP to publish the mapping.

$ arp -s 129.226.55.33 9:6:a:b:3:e pub

4. Check the mapping published by ARP, which should appear as shown on 
the seconds line of this example.

$ arp -a
(129.226.55.33) at 9:6:a:b:3:e permanent published

>From this point onward any system on the network can access this aliased 
IP address (129.226.55.33) using ping, telnet, ftp, rlogin, rcp, rsh, and 
so forth.

guidob (guidob@synnergy.net)
Synnergy Networks (c) 1999, http://www.synnergy.net