This question's explanation and your answers so far.
ハブとルータを使用してPC1～4が相互に通信できるように構成したTCP/IPネットワークがある。ルータの各ポートに設定したIPアドレスが図のとおりであるとき，PC1に設定するデフォルトゲートウェイのIPアドレスとして，適切なものはどれか。 - 左のルータと右のルータが相互に接続されている。左ルータ側のポート：192.168.5.1，右ルータ側のポート：192.168.5.2 - 左ルータのもう一方のポート（192.168.1.1）→ ハブ → PC1，PC2（LAN1） - 右ルータのもう一方のポート（192.168.2.1）→ ハブ → PC3，PC4（LAN2）
正解 A default gateway is the first relay point to which a PC forwards packets when it communicates with a destination outside its own network (the same subnet) — that is, the port of the router on the LAN side to which the PC belongs. In the figure, PC1 belongs to LAN1, which is connected through a hub to the left router's port 192.168.1.1. For PC1 to communicate with something outside LAN1, such as LAN2 (192.168.2.x) or the link between the two routers (192.168.5.x), it must first hand the packet to the left router's port 192.168.1.1, which is on the same segment. Therefore the default gateway to set on PC1 is 192.168.1.1, and the correct answer is ア.
192.168.2.1 is the LAN2-side port of the right router, serving as the gateway for the separate segment to which PC3 and PC4 belong. It is on a different subnet from PC1 and cannot be reached directly by PC1, so it cannot be used as PC1's default gateway.
192.168.5.1 is the port on the left router's side of the inter-router link that connects to the right router. It is on a different segment from LAN1 (192.168.1.x) to which PC1 belongs, so PC1 cannot specify it directly as an address within the same network, and it cannot serve as the gateway.
192.168.5.2 is the port on the right router's side of the inter-router link. It is neither on the same segment as PC1 nor on PC1's own router — it sits on the neighboring router — so it is not PC1's first forwarding destination, i.e., not the default gateway.