if ( ipaddr )
    {
        /*etharp_cached_entry儲存了上次返回的索引*/
        if ( arp_table[etharp_cached_entry].state == ETHARP_STATE_STABLE )
        {
            /*如果上次的還有效，並且是這個要找的ip，直接返回，這個技巧很快*/
            /* the cached entry is stable */
            if ( ip_addr_cmp ( ipaddr, &arp_table[etharp_cached_entry].ipaddr ) )
            {
                /* cached entry was the right one! */
                ETHARP_STATS_INC ( etharp.cachehit );
                return etharp_cached_entry;
            }
        }
    }

/*下面的for做搜尋，並記錄一下表項的狀態和生存時間*/
    for ( i = 0; i < ARP_TABLE_SIZE; ++i )
    {
        /* no empty entry found yet and now we do find one? */
        if ( ( empty == ARP_TABLE_SIZE ) && ( arp_table[i].state == ETHARP_STATE_EMPTY ) )
        {
            /*empty == ARP_TABLE_SIZE
            這個第一次生效會生效，這裡只進入一次，就是記錄第一個出現empty表項的位置*/
            /* remember first empty entry */
            empty = i;
        }
        /* pending entry? */
        else if ( arp_table[i].state == ETHARP_STATE_PENDING )
        {
            /**看ip是否匹配，匹配的話就直接返回索引更新etharp_cached_entry，*/
            /* if given, does IP address match IP address in ARP entry? */
            if ( ipaddr && ip_addr_cmp ( ipaddr, &arp_table[i].ipaddr ) )
            {
                /* found exact IP address match, simply bail out */
                etharp_cached_entry = i;
                return i;
#if ARP_QUEUEING
                /* pending with queued packets? */
            }
            else if ( arp_table[i].q != NULL )
            {
                        /*找生存時間最長的*/
                if ( arp_table[i].ctime >= age_queue )
                {
                    old_queue = i;
                    age_queue = arp_table[i].ctime;
                }
#endif
                /* pending without queued packets? */
            }
            else
            {
                        /*找生存時間最長的*/
                if ( arp_table[i].ctime >= age_pending )
                {
                    old_pending = i;
                    age_pending = arp_table[i].ctime;
                }
            }
        }
        /* stable entry? */
        else if ( arp_table[i].state == ETHARP_STATE_STABLE )
        {
            /* if given, does IP address match IP address in ARP entry? */
            if ( ipaddr && ip_addr_cmp ( ipaddr, &arp_table[i].ipaddr ) )
            {
                /*找到匹配的就直接返回了，就是它了！！*/
                etharp_cached_entry = i;
                return i;
                /* remember entry with oldest stable entry in oldest, its age in maxtime */
            }
            else if ( arp_table[i].ctime >= age_stable )
            {/*沒有就記錄一下它活了多久，一會會有判斷，如果活得太久了，就幹掉它，返回一個empty的給上層*/
                old_stable = i;
                age_stable = arp_table[i].ctime;
            }
        }
    }
    /* { we have no match } => try to create a new entry */

    /* no empty entry found and not allowed to recycle? */
    if ( ( ( empty == ARP_TABLE_SIZE ) && ( ( flags & ETHARP_TRY_HARD ) == 0 ) )
            /* or don't create new entry, only search? */
            || ( ( flags & ETHARP_FIND_ONLY ) != 0 ) )
    {