TCL, MAC address increment

set var_mac 001A6A00001F
set var_incr 000000000001

set var_mac2 [ format %X [ expr "0x$var_mac + 0x$var_incr" ] ]
puts $var_mac2

set mac_next "001A$var_mac2"
puts "Next MAC Address is $mac_next"
[OUTPUT]
6A000020
Next MAC Address is 001A6A000020

[NOTE]
Line 1 to Line 4 產生的值為 Line 5 的 6A000020, 而不是 001A6A00001F, 少了 001A. 很奇怪, 所以才在 Line 7 補上 "001A", 有空再回頭瞭解.
Line 2 $var_incr 也可不用 000000000001, 可以是 1.