Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_19705 ( "Prefix size" text, "Network mask" text, "Available subnets" real, "Usable hosts per subnet" real, "Total usable hosts" real )

### Question ###
When the network mask is 255.255.255.128 what is the lowest available subnet?

### Accurate SQL ###

SELECT MIN("Available subnets") FROM table_19705 WHERE "Network mask" = '255.255.255.128'