а точно ли, что тип у него SNMP_SYNTAX_CNTR32? и по-моему, там строка, и не IP адрес, а именно "физический", то есть MAC.
3.2.5 ifPhysAddress
This object contains the IEEE 802.3 address which is placed in the source-address field of any Ethernet, Starlan, or IEEE 802.3 frames that originate at this interface. Usually this will be kept in ROM on the interface hardware. Some systems may set this address via software.
In a system where there are several such addresses the designer has a tougher choice. The address chosen should be the one most likely to be of use to network management (e.g. the address placed in ARP responses for systems which are primarily IP systems).
If the designer truly can not chose, use of the factory- provided ROM address is suggested.
If the address can not be determined, an octet string of zero length should be returned.
The address is stored in binary in this object. The address is stored in "canonical" bit order, that is, the Group Bit is positioned as the low-order bit of the first octet. Thus, the first byte of a multicast address would have the bit 0x01 set.
http://www.apps.ietf.org/rfc/rfc2358.html#sec-3.2.5