When listing MAC addresses on a Cisco ISR DHCP service, I get a long list of hardware addresses, one per IP address, but for one IP I get an address so long it runs onto three lines. The other addresses are all either 48 bit or 56 bit. What do I make of this long one? Could a device really use this address in a DHCP request? The interfaces are of type Gigabit Ethernet. Could this be a misconfiguration on this switch?

Below is partial readout including the relevant IP.

somerouterName#sho ip dhcp bind                    
Bindings from all pools not associated with VRF:
IP address          Client-ID/          Lease expiration        Type
            Hardware address/
            User name
<SNIP>         01c0.8c71.8b24.11       Oct 08 2019 02:20 PM    Automatic         c42f.ad16.1012          Oct 08 2019 01:42 PM    Automatic         ff50.fe6e.6100.0200.    Oct 08 2019 01:42 PM    Automatic
                    6aea.46         01bc.5ff4.4b99.7a       Oct 08 2019 01:41 PM    Automatic
  • 2
    client-id doesn't have to be a MAC.
    – Ricky
    Oct 8, 2019 at 1:54
  • 2
    DHCP can assign by Client ID or a user value, too. Notice that the heading is "Client-ID/Hardware address/User name." Also, not all hardware addresses are MAC addresses, and there are 48-bit and 64-bit MAC addresses.
    – Ron Maupin
    Oct 8, 2019 at 1:58
  • 1
    By the way, I count 152 bits, not 88 bits in that long string.
    – Ron Maupin
    Oct 8, 2019 at 2:00
  • I had 152 bit also. Dunno why I changed it to 88, thanks.
    – thorr18
    Oct 8, 2019 at 2:14
  • 1
    According to RFCs, DHCP client-id can contain a few MACs or/and other options (depend on OS and system configuration). Server does not care - all it needs that client-ids are the different. You maybe can check the format of DHCP client-id for particular software/hardware of machine. But it is not a problem anyway. Oct 8, 2019 at 9:04

1 Answer 1


DHCP mostly works now-days using client-ID, which is a sequence usually but not mandatory based on the MAC address of a device.

For devices like CISCO's switched that hand-out DHCP allocations there's just the shifted MAC address (by default). In your example:

01c0.8c71.8b24.11 comes from shifted c08c.718b.2411, which is the original address and represents hardware type (01) and client hardware address (the MAC).

This is a default, but can be changed (ip dhcp client client-id allows you to define your own format and add an ASCII string into client-id ).

When actual MACs are used, like c42f.ad16.1012 in your example, usually this is in the case of very old devices (like old printers) which don't properly support client-ID. If a DHCP client sends no DHCP Client Identifier option, the service continues to operate as it has in the past, based only on the hardware address.

Now, in the case of special clients, things may differ. Client-ID may contain another type of identifier, such as a DNS name or even a next hop classic client-ID. The 'client identifier' chosen by a DHCP client only MUST be unique to that client within the subnet to which the client is attached, it does not have to mandatory be under the format used preferably by CISCO (hardware type and client hardware address).

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