the absolute max session id is 999_999_999
which in binary is:

$ perl -e 'printf"%032b$/",999999999'
00111011100110101100100111111111

you have the 2 most significant bits to work with.

%NASs = ( '192.168.254.12' => 0, '192.168.255.13' => 1,
  '192.168.256.2' => 2, '192.168.243.1' => 3 );

so you can fit 4 IP's and a session id in 32 bits.

if you want to assume that your session id won't go above 463129087 then you can steal another bit and have 8 IPs.

$ perl -le 'print unpack "N", pack "B*", "00011011100110101100100111111111"'
463129087

or along the same lines, use the NAS number and session id with a dot,
only because it's easier for humans.

 1.4596831
12.4596831

instead of the 16bit big numbers...

##</code><code>##

$ snmpget -v 1 $CENSORED sysDescr.9999999999
Error in packet
Reason: (noSuchName) There is no such variable name in this MIB.
Failed object: system.sysDescr.4294967295

$ snmpget -v 1 $CENSORED sysDescr.4294967294
Error in packet
Reason: (noSuchName) There is no such variable name in this MIB.
Failed object: system.sysDescr.4294967294

$ snmpget -v 1 $CENSORED sysDescr.4294967295
Error in packet
Reason: (noSuchName) There is no such variable name in this MIB.
Failed object: system.sysDescr.4294967295

$ snmpget -v 1 $CENSORED sysDescr.4294967296
Error in packet
Reason: (noSuchName) There is no such variable name in this MIB.
Failed object: system.sysDescr.4294967295