why when spi "on", my client will block do me have to change my modem for work on server or network?

@Kevin_Z thanks for reply ok. when we enable SPI access from outside to the local network get block and every thing work ok but me say: my client is a web server. my site is on this client....

hime use than this product TD-W8951ND V5 me for use hosting have to use "NAT" and some body take 403 forbidden error and no internet conection- internet block and have to restart modem with "Current...