The SoftBank Hikari router, like most routers on the market, has the feature of assigning fixed internal IP addresses based on MAC addresses (in Japanese, it’s termed as ‘固定割当’). The official website does have a related tutorial (the author uses the ‘光BBユニット2.4’).

As mentioned above, to assign an IP address to a specific device’s MAC address, one needs to go to ‘ルーター機能の設定’ (Router Function Settings) → ‘IPアドレス/DHCPサーバの設定’ (IP Address/DHCP Server Settings).

In the ‘固定割当’ (Fixed Assignment) field, first ensure that the IP address entered is within the ‘割当IPアドレスの範囲’ (Assigned IP Address Range).

Then, it’s time to enter the MAC address of the device. To check the MAC addresses of each device, a quicker method is to go to the router management backend homepage, find ‘設定/接続情報’ (Settings/Connection Information) and locate ‘DHCPクライアント一覧’ (DHCP Client List):

However, the MAC addresses listed here have Latin letters in lowercase, for example:

70:48:d1:ff:9e:a9

If you copy and paste this directly into the ‘固定割当’ settings interface, it may result in an error:

‘固定割当’の’MACアドレス’に誤りがあります.
(There is an error in the ‘MAC Address’ of ‘Fixed Assignment’.)

You need to change the letters to uppercase for it to work.

Afterward, save the settings. Once all configurations are completed, the router will need 120 seconds to reboot, after which you will find it effective.