Think as subnet as a bucket
You can do one of several things
1. Fill the bucket if it's empty: subnet=value ergo subnet='12345' - string wiyh value 12345
2. Replace current contents same as 1, old value is replaced
3. empty the bucket (example for a string) subnet='' (which is actually the same as 2, with specific value)
4. del subnet - delete subnet. This is rarely needed as garbage collection will take care of it for you.
You can do one of several things
1. Fill the bucket if it's empty: subnet=value ergo subnet='12345' - string wiyh value 12345
2. Replace current contents same as 1, old value is replaced
3. empty the bucket (example for a string) subnet='' (which is actually the same as 2, with specific value)
4. del subnet - delete subnet. This is rarely needed as garbage collection will take care of it for you.